Click on the alphabet and there you go!

H. flu immunization: See HIB immunization.

Haemophilus influenzae (also H. flu): (Not the cause of influenza as it was once thought to be), H. flu is a bacteria capable of causing ear infections, meningitis, cellulitis (soft tissue infection), upper respiratory infections, pneumonia and other infections, especially in young children.

Haemophilus influenzae type B immunization: See HIB immunization,

Hair follicle: A sac from which a hair grows.

Hairy cell leukemia: A rare type of chronic leukemia in which the abnormal white blood cells appear to be covered with tiny hairs. Hammer toe: A flexed (curly) toe but with no abnormal rotation of the toe. May require surgical correction.

Hansen disease: Leprosy.

Haploid: A set of chromosomes with only one member of each chromosome pair. The sperm and egg are haploid and, in humans, have 23 chromosomes.

Hard palate: The bony part of the roof of the mouth. The hard palate is the front of the palate and is in front of the soft palate.

HAVRIX: A vaccine against hepatitis A made of killed hepatitis A virus to stimulate the body’s immune system to produce antibodies against the hepatitis A virus.

Hay fever: Allergic rhinitis.

HBIG: Hepatitis B immune globulin, which contains antibodies to hepatits B virus and offers prompt but short lived protection.

hCG: Human chorionic gonadotropin, a human hormone produced by the fetal part of the placenta that stimulates the gonads. The abbreviation can also be written entirely in capital (upper case) letters as "HCG". For more on this hormone in key lab tests, please see: Human chorionic gonadotropin.

Hct: Hematocrit.

HDL: High-density lipoprotein.

HDL cholesterol: High-density lipoprotein cholesterol (the "good" cholesterol).

Health care proxy: A health care proxy is one form of advance medical directive. Advance medical directives preserve the person’s right to accept or reject a course of medical treatment even after that person becomes mentally or physically incapacitated to the point of being unable to communicate those wishes. There are two basic forms of advance directives: (1) a living will, in which the person outlines specific treatment guidelines that are to be followed by health care providers; (2) a health care proxy (also called a power of attorney for healthcare decision-making) in which the person designates a trusted individual to make medical decisions in the event that he or she becomes too incapacitated to make such decisions. Advance directive requirements vary greatly from one jurisdiction to another and should therefore be drawn up in consultation with an attorney who is familiar with the laws of the particular jurisdiction. (This entry is based upon material from the National MS Society).

Heart: The heart is a muscle which pumps blood it receives from veins into arteries throughout the body. The heart is composed of specialized muscle called "cardiac muscle." (see muscle). The heart, veins and arteries make up the circulatory system.

Heart attack: A coronary artery occlusion or myocardial infarction (MI).

Heart block: A block in the conduction of the normal electrical impulses in the heart.

Heartburn: Heartburn has nothing to do with the heart. It is an uncomfortable feeling of burning and warmth occurring in waves rising up behind the breastbone (sternum) toward the neck. It is usually due to gastroesophageal reflux, the return of stomach acid back up into the esophagus, the soft tube-like portion of the digestive tract connecting the pharynx with the stomach.

Heart failure: Inability of the heart to keep up with the demands on it and pump the blood with normal efficiency.

Heart murmur: An unusual heart sound which may be innocent or reflect disease.

Heart rate: The number of heart beats per unit time, usually per minute. The heart rate is based on the number of contractions of the ventricles (the lower chambers of the heart). The heart rate may be too fast (tachycardia) or too slow (bradycardia). The pulse is bulge of an artery from the wave of blood coursing through the blood vessel as a result of the heart beat. The pulse is often taken at the wrist to estimate the heart rate.

Hecht syndrome: Inherited disorder transmitted as an autosomal dominant trait in which short tight muscles make it impossible to open the mouth fully or keep the fingers straight when the hand is flexed back. The small mouth creates feeding problems. The hands may be so tightly fisted the infant crawls on the knuckles. Also called the trismus pseudocamptodactyly syndrome.

Helicobactor pylori: Bacteria that cause inflammation and ulcers in the stomach.

Hemangioma: A birth irregularity where a localized tissue mass grows rich in small blood vessels. Capillary hemangiomas are composed nearly entirely of tiny capillary vessels. Cavernous hemangiomas are composed of blood-filled "lakes" and channels.

Hemarthrosis: Blood in a joint.

Hematemesis: Bloody vomit.

Hematocrit: The percentage, by volume, of red cells in blood. Normal range for males is about 40-54 and for females 37-47 (values may vary slightly between laboratories).

Hematologist: A doctor who specializes in treating diseases of the blood.

Hematoma: A hematoma is a localized swelling filled with blood. The blood is usually clotted or partially clotted and exists within an organ or a soft tissue space, such as muscle.

Hematopoiesis: The formation and development of blood cells.

Hematuria: Blood in the urine.

Hemiparesis: Weakness on one side of the body.

Hemiplegia: One side of the body is paralyzed.

Hemizygous: Having only a single set of genes as, for example, on the single X chromosome in the male.

Hemodialysis: Filtration and cleansing of the blood. Commonly called dialysis.

Hemoglobin: The oxygen-carrying pigment of red blood cells.

Hemoglobin A: The main type of hemoglobin after early infancy. The A stands for Adult.

Hemoglobin F: The main type of hemoglobin in the fetus and still at birth. The F stands for Fetal.

Hemoglobin S: The most common type of abnormal hemoglobin, hemoglobin S is found in sickle cell trait and sickle cell anemia. It differs from hemoglobin A only by a single amino acid substitution. Recognition of this tiny change marked the opening of molecular medicine.

Hemoglobinuria: Hemoglobin in the urine.

Hemolysis: Breakdown of red blood cells.

Hemolytic anemia: Anemia due to the destruction (rather than underproduction) of red blood cells.

Hemolytic disease of the newborn: Abnormal breakup of red blood cells in the fetus or newborn.

Hemophilia: A set of bleeding disorders.

Hemophilia A: Classic hemophilia (the disease of the Russian royal house and other descendants of Queen Victoria). Due to profound deficiency of factor VIII which is necessary for normal blood clotting. The hemophilia A gene is on the X chromosome so females carry the gene and each of their sons stands a 50% chance of receiving the gene and having hemophilia. Treatment of hemophiliacs with contaminated blood products exposed many to HIV.

Hemophilia B: Also called Christmas disease (so-named for the first patient studied in detail with the disease). Due to deficiency of coagulation factor IX. The hemophilia B gene is also on the X chromosome.

Hemoptysis: Spitting up blood or blood-tinged sputum.

Hemorrhage: Hemorrhage refers to bleeding or a flow of blood. It can be internal, and not be visible, or external, and therefore, visible on the body.

Hemorrhoids: Dilated veins around the rectum.

Heparin: An anticoagulant (anti-clotting) medication. Heparin is useful in preventing thromboembolic complications (clots that travel from their site of origin through the blood stream to clog up another vessel). Heparin is also used in the early treatment of blood clots in the lungs (pulmonary embolisms).

Hepatic: Having to do with the liver.

Hepatitis: Inflammation of the liver.

Hepatitis A: Hepatitis due to the hepatitis A virus which is usually transmitted by poor hygiene. (One reason why food service employees are required to wash their hands after using the toilet.)

Hepatitis A immunization: When immediate protection against hepatitis A (infectious hepatitis) is needed, immunoglobulins are used. Protection is effective only if given within 2 weeks of exposure and lasts but 2-4 months. Immunoglobulins can be used to protect household contacts of someone with acute viral hepatitis and travelers to regions with poor sanitation and high hepatitis A rates, when the traveler has to depart sooner than the vaccines can take effect (about 2 weeks). Travelers can receive the immunoglobulin and vaccine simultaneously and be protected immediately and for longer term. When immediate protection is not needed, hepatitis A vaccines are considered for individuals in high-risk settings, including frequent world travelers, sexually active individuals with multiple partners, homosexual men, individuals using illicit drugs, employees of daycare centers, and certain healthcare workers, and sewage workers. Two hepatitis A vaccines called HAVRIX and VAQTA are commercially available in the U.S. Both are highly effective and provide protection even after only one dose. Two doses are recommended for adults and 3 doses for children (under 18 years of age) to provide prolonged protection.

Hepatitis B: Hepatitis due to the hepatitis B virus once thought to be passed only through blood products and so called serum hepatitis. Now known to be passed also by needle sticks, body piercing and tattooing (if proper sterilization or disposable one-use instruments are not used), dialysis, sexual and even less intimate close contact, and childbirth.

Hepatitis B immunization: Hepatits B (hep B) vaccine gives prolonged protection, but 3 shots over a half year are usually required. In the U.S., all infants receive hep B vaccine. Two vaccines (ENGERIX-B, and RECOMBIVAX-HB) are available in the US. The first dose of hep B vaccine is frequently given while the newborn is in the hospital or at the first doctor visit following birth. The second dose is given about 30 days after the initial dose. A booster dose is performed approximately six months later. Babies born to mothers testing positive for hep B receive, in addition, HBIG (hep B immune globulin) for prompt protection. Older children (11-12 years) are advised to receive a hep B booster as are adults in high-risk situations including healthcare workers, dentists, intimate and household contacts of patients with chronic hep B infection, male homosexuals, individuals with multiple sexual partners, dialysis patients, IV drug users, and recipients of repeated transfusions. Healthcare workers accidentally exposed to materials infected with hep B (such as needle sticks), and individuals with known sexual contact with hep B patients are usually given both HBIG and vaccine to provide immediate and long term protection.

Hepatitis C: Hepatitis due to the hepatitis C virus (HCV) which is usually spread by blood transfusion, hemodialysis, and needle sticks. Causes most transfusion-associated hepatitis. Transmission of the virus by sexual contact is rare. At least half of patients develop chronic hepatitis C infection. Hepatitis C was previously called "non-A, non-B hepatitis.

Hepatitis D, E, F, and G: Lesser known (than hepatitis A, B, and C), the most significant of these seems to be type D, or the delta agent, which only causes disease in the presence of the hepatitis B virus.

Hepatitis, infectious: See Hepatitis A.

Hepatitis, non-A, non-B: The old name for hepatitis C when the causative virus had not been identified but it was known not to be hepatitis A or B.

Hepatitis, viral: Liver inflammation caused by viruses. Specific hepatitis viruses have been labeled A, B, C, D, E, F, and G. While other viruses, such as the mononucleosis (Epstein-Barr) virus and cytomegalovirus, can also cause hepatitis, the liver is not their primary target.

Hepatomegaly: An enlarged liver.

Hepatosplenomegaly: Enlargement of the liver and spleen.

Hepatotoxic: Injurious to the liver. For example, acetaminophen (TYLENOL) can be hepatotoxic.

Herbalist: One versed in herbal lore and, in regard to therapy, an herb doctor.

Heritable: Capable of being transmitted from parent to child.

Heritability: The degree to which something is inherited.

Hereditary angioedema: A genetic form of angioedema. (Angioedema is also referred to as Quinke’s disease.) Persons with it are born lacking an inhibitor protein (called C1 esterase inhibitor) that normally prevents activation of a cascade of proteins leading to the swelling of angioedema. Patients can develop recurrent attacks of swollen tissues, pain in the abdomen, and swelling of the voice box (larynx) which can compromise breathing. The diagnosis is suspected with a history of recurrent angioedema. It is confirmed by finding abnormally low levels of C1 esterase inhibitor in the blood. Treatment options include antihistamines and male steroids (androgens) that can also prevent the recurrent attacks. Also called hereditary angioneurotic edema.

Hereditary angioneurotic edema: A genetic form of angioedema. (Angioedema is also referred to as Quinke’s disease.) Persons with it are born lacking an inhibitor protein (called C1 esterase inhibitor) that normally prevents activation of a cascade of proteins leading to the swelling of angioedema. Patients can develop recurrent attacks of swollen tissues, pain in the abdomen, and swelling of the voice box (larynx) which can compromise breathing. The diagnosis is suspected with a history of recurrent angioedema. It is confirmed by finding abnormally low levels of C1 esterase inhibitor in the blood. Treatment options include antihistamines and male steroids (androgens) that can also prevent the recurrent attacks. Also called hereditary angioedema.

Hereditary mutation: A gene change that occurs in a germ cell (an egg or sperm) to become incorporated in every cell in the body. Hereditary mutations (also called germline mutations) play a role in cancer as, for example, the eye tumor retinoblastoma and Wilms' tumor of the kidney.

Hereditary spherocytosis (HS): Genetic disorder of red blood cells. In HS, the red cells are abnormal in shape. They are spherical rather than the normal svelt biconcave-disk shape. They are also unusually fragile. The rotund HS red cells tend to get trapped in narrow blood passages (such as in the spleen) and break up (hemolyze) easily, leading to anemia.

Heredity: Genetic transmission from parent to child.

Hernia: Also called rupture, "hernia" is a general term referring to a protrusion of a tissue through the wall of the cavity in which it is normally contained.

Hernia, hiatus: Protrusion of the stomach up into the opening normally occupied by the esophagus in the diaphragm, the muscle that separates the chest (thoracic) cavity from the abdomen. Hiatus in Latin means an opening.

Herniation: Abnormal protrusion of tissue through an opening. For example, a intervertebral disk (one situated between the vertebral bodies) can protrude and impinge on a nerve root.

Heroin: Semisynthetic drug derived from morphine. Discovered in 1874, it was introduced commercially in 1898 by the Bayer company in Germany. The name heroin was coined from the German heroisch meaning heroic, strong. Heroin is stronger (more potent) than morphine.

Herpes, genital: A viral infection transmitted through intimate contact with the moist mucous linings of the genitals. This contact can involve the mouth, the vagina or the genital skin. The herpes simplex type 2 virus enters the mucous membranes through microscopic tears. Once inside, the virus travels to nerve the roots near the spinal cord and settles there permanently. When an infected person has a herpes outbreak, the virus travels down the nerve fibers to the site of the original infection and when it reaches the skin, the classic redness and blisters occur. The outbreak of herpes is closely related to the functioning of the immune system. Women who have suppressed immune systems, either through stress, disease, or medications, have more frequent and longer-lasting outbreaks. Commonly just called "herpes."

Herpes simplex type 1: A virus that causes cold sores and fever blisters.

Herpes simplex type 2: Different from herpes simplex type 1, herpes simplex 2 causes genital herpes.

Herpesvirus: A member of the herpes family of viruses. One type of herpesvirus is sexually transmitted and causes sores on the genitals.

Herpes zoster: Also called shingles, zona, and zoster. The culprit is the varicella-zoster virus. Primary infection with this virus causes chickenpox (varicella). At this time the virus infects nerves (namely, the dorsal root ganglia) where it remains latent (lies low) for years. It can then be reactivated to cause shingles with blisters over the distribution of the affected nerve accompanied by often intense pain and itching.

Hetero-: Combining form from the Greek heteros meaning different. The opposite is homo- from the Greek homos meaning same. For example, heterogeneous and homogeneous, heterosexual and homosexual, etc.

Heterochromatin: A genetically inactive part of the genome, heterochromatin was so named because it was chromosomal material (chromatin) that stained differently, more darkly, all through the cell cycle, than most chromosomal material (which was named euchromatin). There are two types, namely constituitive heterochromatin and facultative heterochromatin.

Heterochromatin, constituitive: Heterochromatin that is fixed and irreversible. Regions of constituitive heterochromatin are located at very specific spots in the genome (on chromosomes 1, 9, 16 and the Y chromosome, the tiny short arms of chromosomes 13-15 and 21 and 22, and near the centromeres of chromosomes) and consists of DNA that contains many tandem (not inverted) repeats of a short basic repeating unit (known as satellite DNA).

Heterochromatin, facultative: Heterochromatin that need not always be heterochromatic but has the faculty to return to the normal euchromatic state. The inactive X chromosome is made up of facultatative heterochromatin. When a woman transmits that X to a son, it reverts to euchromatin and genetic activity.

Heterochromia: Different colors.

Heterochromia iridis: A difference of color between the iris of one eye and the other. (A person with one brown and one blue eye has heterochromia iridis.) Also, a difference in color within an iris (sectoral heterochromia iridis).

Heteroerotic: Having to do with sexual excitement toward the opposite sex. By contrast with alloerotic.

Heterokaryon: A cell with two separate nuclei formed by the experimental fusion of two genetically different cells. (Heterokaryons, for example, composed of nuclei from Hurler syndrome and Hunter syndrome, both diseases of mucopolysaccharide metabolism, have normal mucopolysaccharide metabolism proving that the two syndromes affect different proteins and so can correct each other in the heterokaryon).

Heteromorphism: Something different in form. Chromosome heteromorphisms are normal variations in the appearance of chromosomes.

Heteroploid: A different chromosome number than the normal number of chromosomes.

Heterosexual: A person sexually attracted to persons of the opposite sex. The word "straight" has become synonymous with heterosexual. Heterosexual can also be an adjective.

Heterosexuality: Sexuality directed toward someone of the opposite sex.

Heterozygote: An individual with different genes at a particular spot (locus) on a pair of chromosomes. (A heterozygote for cystic fibrosis (CF) has the CF gene on one chromosome 7 and the normal paired gene on the other chromosome 7. That makes them a carrier for CF.)

Heterozygous: The state of being heterozygous.

Hexoseaminidase A: Deficiency of this enzyme causes Tay-Sachs disease, a progressive, fatal neurologic disorder concentrated in people of European Jewish (Ashkenazi) descent.

Hiatus hernia: Protrusion of the stomach up into the opening normally occupied by the esophagus in the diaphragm, the muscle that separates the chest (thoracic) cavity from the abdomen. Hiatus in Latin means an opening.

HIB: Haemophilus influenzae type b.

HIB immunization: This vaccine is to prevent disease caused by the Haemophilus influenzae type B (HIB) bacteria. The H. influenzae (H. flu) bacteria can cause a range of serious diseases including meningitis with potential brain damage and epiglottitis with airway obstruction poisoning. The HIB vaccine is usually given at 2, 4 and 6 months of age. A final booster is given at 12-15 months of age. HIB vaccine rarely causes severe reactions.

Hiccups: Spasmodic contractions of the diaphragm that are involuntary and often rhythmic. Usually just a minor nuisance, prolonged hiccups can become a major problem.

Hidradenitis suppurativa: This is an illness characterized by multiple abscesses that form under the arm pits and in the groin area.

High blood pressure: A repeatedly elevated blood pressure exceeding 140 over 90 mmHg. High blood pressure (hypertension) is "the silent killer." Chronic high blood pressure can stealthily cause blood vessel changes in the back of the eye (retina), abnormal thickening of the heart muscle, kidney failure, and brain damage. No specific cause for high blood pressure is found in 95% of patients. High blood pressure is treated with salt restriction, regular aerobic exercise, and medications.

Hippocampus: An area buried deep in the forebrain that helps regulate emotion and memory.

Hip bursitis: A bursa is a fluid-filled sac that functions as a gliding surface to reduce friction between moving tissues of the body. There are two major bursae of the hip. Bursitis is usually not infectious, but the bursa can become infected. Treatment of non-infectious bursitis includes rest, ice, and medications for inflammation and pain. Infectious bursitis is treated with antibiotics, aspiration, and surgery.

Hip pointer: Sportstalk for an iliac crest contusion (a bruise of the upper edge of the ilium, one of the hip bones).

Hirschsprung's disease: Absence of nerves (ganglia) in the bowel wall starting in the anus and extending up a variable distance with enlargement of the bowel above that point. Hirschsprung's disease is the commonest cause of lower intestinal obstruction in the newborn and, later, one of the causes of chronic constipation. Also called congenital aganglionic megacolon.

Hirsute: Overly hairy.

Hirudin: An anticoagulant (anti-clotting) agent that prevents thromboembolic complications (clots that travel through the blood stream to clog up a vessel). Hirudin is the active principle in the secretion of leeches. Desirudin and lepirudin (REFLUDAN) are genetically engineered recombinant forms of hirudin.

Histoplasma: A fungus found worldwide. In the USA, it is so common in the Midwest that in parts of Kentucky and Tennessee nearly 90% of adults show evidence of exposure (with a positive histoplasma skin test).

Histoplasmosis: Infection with histoplasma. Most patients have no symptoms However, histoplasma can cause acute or chronic lung disease or progessive disseminated histoplasmosis (a particular hazard for persons with HIV).

Histamine: Substance that plays a major role in many allergic reactions. Histamine dilates blood vessels and makes the vessel walls abnormally permeable.

Histocompatible: The prefix histo- means tissue. The term histocompatible is literally tissue compatible. If a donor and recipient are histocompatible (like identical twins), a transplant will be easily accepted.

Histones: Proteins associated with DNA in chromosomes.

History: In medicine, the patient's past and present which may hopefully contain clues helpful to their future health.

HIV: Acronym for the Human Immunodeficiency Virus, the cause of AIDS.

Hives: Urticaria. Raised, itching areas of skin, often a sign of an allergic reaction. Also called "welts" or "nettle rash."

HLA: The HLA complex is the major human histocompatibility system. HLA-typing is done before transplantation to determine the degree of histocompatability. HLA is an acronym for Human Leukocyte Antigens).

Hodgkin's disease (Hodgkin's lymphoma): A disease of the lymph nodes named after the English physician Thomas Hodgkin (1798-1866) who discovered it. ("Perfecting the World" is an excellent biography of Dr. Hodgkin by A. M. and E. H. Kass).

Homeobox: A short stretch of nucleotides (DNA or RNA) with an almost identical base sequence in all genes that contain that stretch. Homeoboxes occur in many organisms from fruit flies to human beings and appear to determine when particular groups of genes are expressed during development.

Homeopath: A person who practices homeopathy.

Homeopathy: A system of therapy founded in the 19th century based on the concept that disease can be treated with drugs (in minute doses) thought capable of producing the same symptoms in healthy people as the disease itself.

Homocystinuria: A genetic disease due to an enzyme deficiency. Among other events, there is a buildup of the amino acid homocystine. Progressive mental retardation is common in untreated cases. The finding of vascular disease and premature arteriosclerosis in persons with homocystinuria led to the theory that homocystine may be a factor in heart disease.

Homolog (homologue): One chromosome of a pair.

Homologies: Similarities in DNA or protein sequences between individuals or between species.

Homologous: The relationship between two chromosomes that are paired and so are homologs of each other.

Homologous chromosomes: A pair of chromosomes containing the same gene sequences, each derived from one parent.

Homosexual: A person sexually attracted to persons of the same sex. Homosexuals include males (gays) and females (lesbians). Homosexual can also be an adjective.

Homosexuality: Sexuality directed toward someone of the same sex.

Hormone: Chemical substance produced in the body that controls and regulates the activity of certain cells or organs. A hormone originally denoted a chemical made by a special gland for export to another part of the body. Now a hormone is more broadly any chemical, irrespective of whether it is produced by a special gland or not, for export or domestic use, that "controls and regulates the activity of certain cells or organs.". The word "hormao" which means "I set in motion" or "I stir up" was used in ancient Greece to covey the "vital principle" of "getting the juices flowing." The word "hormone" was resurrected in 1902 (not 1906, as the Oxford English Dictionary states) by the English physiologists Wm. M. Bayliss and Ernest H. Starling who that year reported their discovery of a substance made by glands in the small intestine that stimulated pancreatic secretion. They called the substance "secretin" and dubbed it a "hormone", the first known hormone.

Hormone, aldosterone: Hormone produced by the outer portion (cortex) of the adrenal gland that regulates the balance of water and electrolytes (ions such as potassium and sodium) in the body. Aldosterone encourages the kidney to excrete potassium into the urine and to retain sodium, thereby retaining water. Aldosterone is classified as a mineralocorticoid hormone.

Hormone, androgenic: Any hormone that promotes the development and maintenance of male sex characteristics. Testosterone is an androgen.

Hormone, cortisol: The primary stress hormone. Cortisol is the major natural glucocorticoid in humans.

Hormone, estrogenic: A female hormone produced by the ovaries (or an equivalent hormone synthesized in the laboratory). Estrogen deficiency can lead to osteoporosis.

Hormone, glucocorticoid: A hormone that predominantly affects the metabolism of carbohydrates and,to a lesser extent, fats and proteins (and has other effects). Glucocorticoids are made in the outside portion (the cortex) of the adrenal gland and chemically classed as steroids. Cortisol is the major natural glucocorticoid. The term glucocorticoid also applies to equivalent hormones synthesized in the laboratory.

Hormone, human chorionic gonadotropin (hCG): A human hormone made by chorionic cells (in the fetal part of the placenta), hCG is directed at the gonads and stimulates them. hCG becomes detectable (by immunologic means) within days of the time of fertilization. It therefore forms the foundation of most common pregnancy tests. The level of hCG in maternal serum enters as one component in the "double" and the "triple" screens used during pregnancy to assign risks of Down syndrome and other fetal disorders.

Hormone, mineralocorticoid: A group of hormones, the most important being aldosterone, that regulate the balance of water and electrolytes (ions such as sodium and potassium) in the body. The mineralocorticoid hormones act specifically on the tubules of the kidney.

Hormone, parathormone: Hormone made by the parathyroid gland (behind the thyroid gland in the neck). Parathormone (pronounced para-thor-mone) is critical to calcium and phosphorus balance. Deficiency of parathormone results in abnormally low calcium in the blood (hypocalcemia). Also call parathyrin.

Hormone, parathyrin: See Hormone, parathormone.

Hormone, parathyroid: See Hormone, parathormone.

Hormone, progesterone: A female hormone, progesterone is the principal progestational hormone. Progestational hormones prepare the uterus (the womb) to receive and sustain the fertilized egg.

Hormone, secretin: Hormone made by glands in the small intestine that stimulates pancreatic secretion. The word "hormone" was coined by the English physiologists Wm. M. Bayliss and Ernest H. Starling in connection with their discovery of secretin, the first hormone, in 1902.

Hormone, T3: Triiodothyronine, a thyroid hormone. The number 3 is usually in subscript.

Hormone,T4: Thyroxine, a thyroid hormone. The number 4 is usually in subscript.

Hormone therapy: A form of treatment that takes advantage of the fact that certain cancers depend on hormones to grow. The treatment may include giving hormones to the patient or decreasing the level of hormones in the body.

Hormone, thyroid: Chemical substance made by the thyroid gland which is located in the front of the neck. The thyroid gland uses iodine to make thyroid hormones. The two most important thyroid hormones are thyroxine (T4) and triiodothyronine (T3).

Hormone, thyroid stimulating (TSH): A hormone produced by the pituitary gland (at the base of the brain) that promotes the growth of the thyroid gland (in the neck) and stimulates it to produce its thyroid hormones. Normally, the rate of thyroid hormone production is controlled by the pituitary. When there are insufficient thyroid hormones in the body for normal functioning of the cells, the pituitary releases TSH. TSH in turn "stimulates" the thyroid gland to produce more thyroid hormones. In contrast, when there is excessive amount of thyroid hormones, the pituitary gland stops producing TSH. The TSH level then falls and thyroid hormone production is reduced. This mechanism maintains a relatively constant level of thyroid hormones circulating in the blood. This phenomenon is analogous to a thermostat used for temperature regulation in a room: when the temperature rises, the thermostat shuts the heater off and the room temperature falls back to normal. High levels of thyroid hormones cause the TSH level to fall, resulting in no further stimulation of the thyroid gland. In hyperthyroidism, there are continuously elevated levels of the thyroid hormones. TSH is also known as thyrotropin.

Hormone, thyrotropin: A hormone produced by the pituitary gland (at the base of the brain) that promotes the growth of the thyroid gland (in the neck) and stimulates it. The suffix -tropin indicates "an affinity for". Thyrotropin has an affinity for the thyroid. Thyrotropin is known also as thyroid stimulating hormone (TSH).

Hormone, thyroxine: A chemical substance made by the thyroid gland The thyroid gland uses iodine to make thyroid hormones. Thyroxine (T4), one of the most important thypoid hormones, has four iodine molecules attached to its molecular structure.

Hormone, triiodothyronine: A hormone made by the thyroid gland. The thyroid gland uses iodine to make thyroid hormones. Triiodothyronine (T3) has three iodine molecules attached to its molecular structure. Iodine is found in seafood, bread, seaweed, and ordinary table salt.

Hormone, TSH: Stands for Thyroid Stimulating Hormone. Also known as thyrotropin.

Horner syndrome: A complex of abnormal findings, namely sinking in of one eyeball, ipsilateral ptosis (drooping of the upper eyelid on the same side) and miosis (constriction of the pupil of that eye) together with anhidosis (lack of sweating) and flushing of the affected side of the face. Due to paralysis of certain nerves (specifically, the cervical sympathetic nerves). Also called Horner-Bernard syndrome, Bernard syndrome, Bernard-Horner syndrome and Horner’s ptosis (but best known as Horner syndrome).

Hornet stings: Stings from hornets and other large stinging insects such as bees, yellow jackets and wasps can trigger allergic reactions varying greatly in severity. Avoidance and prompt treatment are essential. In selected cases, allergy injection therapy is highly effective. (The three "A’s" of insect allergy are Adrenaline, Avoidance and Allergist.)

HPV: Human papilloma virus.

Ht: Abbreviation for height (and also heart).

HUGO: Human Genome Organization (the international organization concerned with human genome research).

Human chorionic gonadotropin (hCG): A human hormone made by chorionic cells (in the fetal part of the placenta), hCG is directed at the gonads and stimulates them. hCG becomes detectable (by immunologic means) within days of fertilization and forms the foundation of the common pregnancy tests. The level of hCG in maternal serum also enters as one component in the "double" and the "triple" screens used during pregnancy to assign risks of Down syndrome and other fetal disorders.

Human gene therapy: Insertion of normal DNA directly into cells to correct a genetic defect. The treatment of disease by replacing, altering, or supplementing a gene that is absent or abnormal that is responsible for the disease. In gene therapy for cancer, for example, researchers are trying to bolster the body’s natural capacity to combat cancer and make the tumor more sensitive to other kinds of therapy. Gene therapy, still in its early stages, holds great promise for the treatment of many diseases.

Human immunodeficiency virus: See HIV.

Human papilloma virus (HPV): A family of over 60 viruses responsible for causing warts. The majority of the viruses produce warts on the hands, fingers, and even the face. Most of these viruses are innocuous, causing nothing more than cosmetic concerns. Several types of HPV are confined primarily to the moist skin of the genitals, producing genital warts and elevating the risk for cancer of the cervix. The papilloma viruses that cause wartlike growths on the genitals are sexually transmitted.

Humerus: The long bone in the upper arm which extends from the shoulder to the elbow.

Humidifier: A machine that puts moisture in the air.

Humor: In medicine, humor refers to a fluid (or semifluid) substance. Thus, the aqueous humor is the fluid normally present in the front and rear chambers of the eye.

Humoral: Pertaining to elements in the blood or other body fluids.

Human genome: The full collection of genes in a human being.

Human Genome Project: International effort aimed at identifying and sequencing (ordering) all of the bases in the human genome. American participation in this monumental undertaking has been supported by funds from the National Institutes of Health (NIH) and the Department of Energy (DOE).

Huntington's disease: An hereditary disorder with mental and physical deterioration leading to death. Although characterized as an "adult-onset" disease (as is usually the case), we have seen children with full-blown Huntington's disease.

Hurler syndrome: A genetic error of metabolism. There is incomplete breakdown and accumulation of a substance (a mucopolysaccharide) which is abnormally stored in the brain and other places. This usually leads to death of the individual with Hurler syndrome by their early teen years. See gargoylism.

Hybrid: The result of a cross between genetically unlike parents.

Hybridoma: A cell hybrid resulting from the fusion of a cancer cell and a normal lymphocyte (a type of white blood cell). The hybridoma is immortal in the laboratory and makes the same products as its parent cells forever.

Hydatid (hydatidiform) mole: An abnormal pregnancy without a placenta or embyro that eventuates in a mass of cysts resembling a bunch of grapes.

Hydrocele: Accumulation of fluid in the coat around the testis. Small hydroceles tend to disappear by a year of age while larger hydroceles may persist and warrant surgery.

Hydrocephalus: Abnormal buildup of cerebrospinal fluid in the ventricles of the brain. The fluid is often under increased pressure and can compress and damage the brain. Treatment is by insertion of a shunt to let the excess fluid exit and relieve the pressure.

Hydrocephaly: See hydrocephalus.

Hydronephrosis: Distention of the kidney with urine. Due to obstruction of urine outflow (for example, by a stone blocking the ureter, the tube going from the kidney to the bladder).

Hymen: A thin membrane which completely or partially occludes the vaginal opening.

Hyper-: Prefix meaning high, beyond, excessive, above normal. For example, hypercalcemia is high calcium in the blood.

Hyperadrenocorticism: Excess hormone called "cortisol". Often called Cushing’s syndrome, it is an extremely complex condition that involves many areas of the body. It results from an excess of cortisol and its effects on the human body. Common symptoms are thinning of the skin, weakness, weight gain, bruising, hypertension, diabetes, weak bones (osteoporosis), facial puffiness, and in women cessation of periods. One of the commonest causes of Cushing’s syndrome is the administration of "cortisol-like medications" for the treatment of diverse diseases. All other cases of Cushing’s syndrome are due to excess production of cortisol by the adrenal gland including 1) an abnormal growth of the pituitary gland, which stimulates the adrenal gland, 2) a benign or malignant growth within the adrenal gland itself, which produces cortisol and 3) production within another part of the body (ectopic production) of a hormone that directly or indirectly stimulates the adrenal gland to make cortisol. Harvey Cushing (1869-1939), a neurosurgeon, described hyperadrenocorticism due specifically to an ACTH-secreting basophilic pituitary adenoma, a benign pituitary tumor that puts out ACTH (AdrenoCorticoTropic Hormone) that, in turn, drives (or overdrives) the adrenal gland.

Hyperaldosteronism: Overproduction of the hormone aldosterone from the outer portion (cortex) of the adrenal gland or a tumor containing that type of tissue. Excess aldosterone (pronounced al-do-ster-one) results in low potassium levels (hypokalemia), underacidity of the body (alkalosis), muscle weakness, excess thirst (polydipsia), excess urination (polyuria), and high blood pressure (hypertension). Also called aldosteronism and Conn’s syndrome.

Hyperbilirubinemia: An elevated level of the pigment bilirubin in the blood. A sufficient elevation will produce jaundice. Some degree of hyperbilirubinemia is very common in babies right after birth, especially premies.

Hypercalcemia: A higher-than-normal level of calcium in the blood. This can cause a number of nonspecific symptoms, including loss of appetite, nausea, thirst, fatigue, muscle weakness, restlessness, and confusion. Excessive intake of calcium may cause muscle weakness and constipation, affect the conduction of electrical impulses in the heart (heart block) lead to calcium stones (nephrocalcinosis), in the urinary tract, impair kidney function, and interfere with the absorption of iron predisposing to iron deficiency. According to the National Academy of Sciences, adequate intake of calcium is 1 gram daily for both men and women. The upper limit for calcium intake is 2.5 grams daily.

Hypercholesterolemia: High blood cholesterol. See familial hypercholesterolemia.

Hyperglycemia: Elevated level of the sugar glucose in the blood.

Hyperkalemia: Elevated blood potassium.

Hyperlipidemia: High lipid (fat) levels in the blood.

Hypermagnesemia: Excess magnesium. Persons with impaired kidney function should be especially careful about their magnesium intake because they can accumulate magnesium, a dangerous (and sometimes fatal) situation. According to the National Academy of Sciences, the Recommended Dietary Allowances of magnesium are 420 milligrams per day for men and 320 milligrams per day for women. The upper limit of magnesium as supplements is 350 milligrams daily, in addition to the magnesium from food and water. Too much magnesium is hypermagnesemia.

Hypermobility syndrome: A common benign childhood condition involving hypermobile joints (that can move beyond the normal range of motion). Symptoms include pains in knees, fingers, hips, and elbows. The affected joints may sprain or dislocate. Scoliosis (curvature of the spine) is more frequent. Usually improves with adulthood. Also called the joint hypermobility syndrome.

Hypernatremia: Elevated blood sodium.

Hyperphosphatemia: A higher than normal blood level of phosphate. Phosphate molecules are particularly important as part of larger molecules in cell energy cycles. Higher than normal levels can be caused by ingestion of phosphate rich foods (diary products) or kidney failure.

Hyperpigmented: Overly pigmented.

Hyperplasia: A precancerous condition in which there is an increase in the number of normal cells lining the uterus.

Hypertension: High blood pressure, defined as a repeatedly elevated blood pressure exceeding 140 over 90 mmHg. High blood pressure (hypertension) is "the silent killer." Chronic high blood pressure can stealthily cause blood vessel changes in the back of the eye (retina), abnormal thickening of the heart muscle, kidney failure, and brain damage. No specific cause for high blood pressure is found in 95% of patients. High blood pressure is treated with salt restriction, regular aerobic exercise, and medications.

Hyperthermia: Treatment that involves heating a tumor.

Hyperthyroid: Excess of thyroid hormone resulting from an overactive thyroid gland (or taking too much thyroid hormone).

Hypertonia: Increased tone of skeletal muscles. Basically, too tight muscles.

Hypertonic solution: One with more salt than in normal cells and blood.

Hyperuricemia: Abnormally elevated blood level of uric acid. Uric acid is a breakdown product of purines that are part of many foods we eat. While hyperuricemia may indicate an increased risk of gout, the relationship between hyperuricemia and gout is unclear. Many patients with hyperuricemia do not develop gout, while some patients with repeated gout attacks have normal or low blood uric acid levels. Among the male population in the United States, approximately ten percent have hyperuricemia. However, only a small portion of those with hyperuricemia will actually develop gout.

Hyperventilation: Overbreathing. Due to anxiety. Overbreathing causes dizziness, lightheadedness, a sense of unsteadiness and tingling around the mouth and fingertips. Relief can be gotten by breathing in and out of a paper bag (to increase the level of carbon dioxide). Opposite, hypoventilation or underbreathing.

Hypo-: Prefix meaning low, under, beneath, down, below normal. For example, hypocalcemia is olow calcium in the blood.

Hypocalcemia: Lower-than-normal blood calcium. Low blood calcium makes the nervous system highly irritable with tetany (spasms of the hands and feet, muscle cramps, abdominal cramps, overly active reflexes, etc.) Chronic calcium deficiency contributes to poor mineralization of bones, soft bones (osteomalacia) and osteoporosis; and, in children, rickets and impaired growth. Food sources of calcium include dairy foods, some leafy green vegetables such as broccoli and collards, canned salmon, clams, oysters, calcium-fortified foods, and tofu. According to the National Academy of Sciences, adequate intake of calcium is 1 gram daily for both men and women. The upper limit for calcium intake is 2.5 grams daily.

Hypoglycemia: Low level of the sugar glucose in the blood.

Hypokalemia: Low blood potassium.

Hypomagnesemia: Too little magnesium. Magnesium deficiency can occur due to inadequate intake or impaired intestinal absorption of magnesium. Low magnesium (hypomagnesemia) is often associated with low calcium (hypocalcemia) and low potassium (hypokalemia). Deficiency of magnesium causes increased irritability of the nervous system with tetany (spasms of the hands and feet, muscular twitching and cramps, spasm of the larynx, etc.). According to the National Academy of Sciences, the Recommended Dietary Allowances of magnesium are 420 milligrams per day for men and 320 milligrams per day for women. The upper limit of magnesium as supplements is 350 milligrams daily, in addition to the magnesium from food and water.

Hyponatremia: Low blood sodium.

Hypoplasia of the thymus and parathyroids: Also known as the DiGeorge syndrome (DGS), this disorder is characterized by (1) low blood calcium levels (hypocalcemia) due to underdevelopment (hypoplasia) of the parathyroid glands needed to control calcium; (2) underdevelopment (hypoplasia) of the thymus, an organ behind the breastbone in which lymphocytes mature and multiply; and (3) defects of the outflow tracts from the heart. Most cases of DGS are due to a microdeletion in chromosome band 22q11.2. A small number of cases have defects in other chromosomes, notably 10p13. Named after the American pediatric endocrinologist Angelo DiGeorge. Another name for DGS is the third and fourth pharyngeal pouch syndrome (since the faulty structures in DGS are embryologically derived from the third and fourth pharyngeal pouches).

Hypophosphatemia: A less than normal blood level of phosphate. The opposite of hyperphosphatemia.

Hypopigmented: Underpigmented.

Hypoplasia: Underdevelopment or incomplete development of a tissue or organ.For example, there can be hypoplasia (underdevelopment) of the enamel of the teeth. Hypoplasia is less drastic than aplasia where there is no development at all.

Hypotension: Low (abnormally low) blood pressure. Hypotension is a consistent finding in shock but is also found in other conditions and so is not necessarily diagnostic of shock. The word hypotension is a hybrid of the Greek "hypo" meaning "under" and the Latin "tensio" meaning "to stretch." In French, "la tension" is "the blood pressure."

Hypotension, orthostatic: Some symptoms of dizziness such as wooziness, feeling about to black out, and tunnel vision can be due to insufficient blood flow to the brain. The cause is transient low blood pressure (hypotension) due usually to suddenly standing up (orthostatic). The symptoms are typically worse when standing, improve with lying down and may be experienced by healthy individuals who rise quickly from a chair, often after a meal, and have a few seconds of disorientation.

Hypothalamus: The area of the brain that controls body temperature, hunger, and thirst.

Hypothermia: Abnormally low body temperature. Someone who falls asleep in a snowbank may become hypothermic. Hypothermia is intentionally produced to slow the metabolism during some types of surgery.

Hypothyroid: Deficiency of thyroid hormone.

Hypotonia: Decreased tone of skeletal muscles. In a word, floppiness.

Hypotonic solution: One with less salt than in normal cells and blood.

Hypovolemia: Abnormal decrease in blood volume (strictly speaking, in the blood plasma).

Hypovolemic shock: See shock.

Hypoxia: Concentration of oxygen in arterial blood that is less than normal. Anoxia refers to complete lack of oxygen.

Hypoxia-ischemia: See hypoxia. Ischemia refers to blood flow to cells and organs that is not sufficient to maintain their normal function.

Hypoxic-ischemic encephalopathy: Damage to cells in the central nervous system (the brain and spinal cord) from inadequate oxygen. Hypoxic-ischemic encephalopathy allegedly may cause in death in the newborn period or result in what is later recognized as developmental delay, mental retardation, or cerebral palsy. This is an area of considerable medical and medicolegal debate.

Hysterectomy: An operation to remove the uterus and sometimes also the cervix.

Hysterectomy, abdominal: Surgical removal of the uterus through an incision made in the abdominal wall. As opposed to a vaginal hysterectomy.

Hysterectomy, complete: Complete surgical removal of the uterus and cervix. Also called a total hysterectomy.

Hysterectomy, partial: The uterus is sugically removed but the cervix is left is left in place. Also called a subtotal hysterectomy.

Hysterectomy, subtotal: The uterus is surgically removed but the cervix is left is left in place. Also called a partial hysterectomy.

Hysterectomy, total: Complete surgical removal of the uterus and cervix. Also called a complete hysterectomy.

Hysterectomy, vaginal: Removal of the uterus through a surgical incision, not of the abdomen but, within the vagina. With a vaginal hysterectomy, the scar is not outwardly visible. A vaginal hysterectomy is in contrast to an abdominal hysterectomy.