Medical Dictionary: H

hair cells: Sensory cells in the inner ear.

hammertoe: A sometimes painful condition in which the toe curls up and under (resembling a hammer).

Hashimoto’s thyroiditis: An autoimmune condition that is the leading cause of hypothyroidism in the United States. Two types of antibodies, called antithyroid peroxidase (anti-TPO) and antithyroglobulin (anti-Tg), promote the destruction of thyroid cells, making them unable to produce adequate amounts of thyroid hormone.

hay fever (seasonal rhinitis): Seasonal allergies to common inhaled allergens.

health care proxy: A legal document that allows an agent to represent a person who is too incapacitated or ill to make health care decisions.

health care proxy: An advance care directive that allows you to appoint someone who will make medical decisions for you if you become incapacitated. Also known as a durable power of attorney for health care.

hearing aid: An electronic device that amplifies sound in the ear, allowing a person to hear better.

heart attack: Damage to, or death of, heart muscle because of insufficient blood supply; the medical term is myocardial infarction.

heart failure: A condition in which the heart loses its ability to efficiently pump blood throughout the body.

Heberden’s node: A bony growth on the joint nearest the fingertip, caused by osteoarthritis.

Heberden’s nodes: bony and fibrous nodules that may form in the end joints of one or more fingers in people with osteoarthritis.

heel spur: An abnormal growth of bone or calcium on the back or bottom of the heel bone.

helper T cells: A subset of T cells. Th2 cells favor the inappropriate immune response seen in allergic reactions.

hematoma: A swelling formed of blood. Infrequently occurs at the site of surgery, such as in a biopsy cavity after a lumpectomy.

hemianopia: Defective vision or blindness in one side of the visual field in one or both eyes; may be caused by an embolic or low-flow transient ischemic attack or stroke in the posterior cerebral artery region.

hemiparesis: Muscular weakness on one side of the body; if the right side is affected, the stroke damaged the left side of the brain.

hemiplegia: Paralysis limited to one side of the body.

hemodialysis: Treatment of kidney failure by cleansing blood outside the body with a hemodialyzer machine, which acts as an artificial kidney.

hemorrhagic stroke: A type of stroke that occurs when a blood vessel ruptures, cutting off the supply of oxygen and nutrition to part of the brain.

HEPA filter: High-efficiency particulate air filter, found in vacuums or in stand-alone devices.

heparin: An anticoagulant drug that inhibits blood from clotting by interfering with coagulation factors; usually administered in the hospital, either by injection or an intravenous line.

herd immunity: Protection of an entire community that occurs when a critical number of community members are immune to an infectious disease. Also known as community immunity.

herniated disk: A displacement of some portion of the intervertebral disk from its normal location; sometimes indicates a ruptured or slipped disk.

herniated disk: Displacement of some portion of the disk out of its normal location.

hertz: The measurement of a sound’s frequency, expressed in cycles per second.

high blood pressure: A longstanding increase in blood pressure above the normal range — that is, abnormally high pressure of blood flow against the artery walls; also called hypertension.

high-density lipoproteins (HDLs): Spherical particles that transport cholesterol from body cells to the liver and other sites for elimination; called “good” cholesterol because high levels are associated with a low risk for heart disease.

hippocampus: Part of the brain that plays a central role in processing long-term memories. It is smaller than normal in some people with post-traumatic stress disorder.

histamine: A substance released by mast cells that causes symptoms of an immediate allergic reaction.

homeostasis: Maintenance of the body’s internal environment through regulation of blood pressure, temperature, water, and nutrient content.

homocysteine: High blood levels of this amino acid have been associated with an increased risk for heart disease. B vitamins appear to reduce blood levels of homocysteine.

hormone therapy: Augmentation of a woman’s depleted hormones with estrogen alone or a combination of estrogen and progestin or progesterone; a measure to reduce osteoporosis risk in postmenopausal women.

hot flash: A sudden feeling of heat in the face or upper part of the body, caused by the dilation of blood vessels in the skin and often accompanied by perspiration and flushing.

HPA (hypothalamic-pituitary-adrenal) axis: A system that governs many hormonal activities in the body, including the stress response.

human immunodeficiency virus (HIV): The virus that causes AIDS.

human leukocyte antigen (HLA) complex: A type of receptor on cells involved in recognizing foreign antigens; these receptors are genetically determined, and some are associated with different types of arthritis.

hyaluronic acid: A hydrating sugar secreted by cells; it’s the predominant substance in the dermis. Hyaluronic acids used as injectable fillers include Restylane, Hylaform, Perlane, Juvaderm, and Captique.

hydrogenation: The addition of hydrogen to a compound, particularly to solidify unsaturated oils.

hydroxyapatite: The mature, hard, somewhat crystalline mineral compounds in bone tissue.

hygiene hypothesis: A suggested explanation for the increase in allergies in industrialized countries. The hypothesis proposes that modern-day cleanliness results in less early exposure to germs and this upsets the balance of the immune response to allergens.

hyperglycemia: An abnormally high level of glucose in the blood.

hyperopia: An optical error in which light rays reach the retina before converging at a focus point; commonly known as farsightedness.

hyperparathyroidism: A condition in which the body produces excessive amounts of parathyroid hormone (PTH), disrupting the regulation of calcium. As a result, calcium is taken from the bones, blood levels of calcium rise, and increased amounts of calcium may be excreted in urine.

hyperplasia: Excessive cell growth.

hyperplastic polyp: A polyp that contains an excess number of normal-looking cells. This is the most common type of polyp found in the colon and rectum.

hypertension: High blood pressure; a major risk factor for stroke, especially hemorrhagic and lacunar strokes, because it puts excess stress on the walls of blood vessels and damages their delicate inner lining. Individuals who are overweight or have obesity are at greater risk of developing high blood pressure.

hypertensive cerebellar hemorrhage: A type of hemorrhagic stroke caused by high blood pressure in which blood leaks into the cerebellum; symptoms include abrupt dizziness and loss of balance.

hypertensive intracerebral hemorrhage: A type of hemorrhagic stroke in which high blood pressure causes a blood vessel deep within the brain to rupture.

hyperthyroidism.

hyperthyroidism: A disease that occurs when the thyroid produces more thyroid hormone than your body needs, thereby abnormally speeding up your metabolism.

hypnagogic hallucinations: Often terrifying dream-like sounds or images occurring just prior to sleep; a symptom of narcolepsy that can be mistaken for psychosis.

hypnogram: A diagram that summarizes the stages of sleep recorded in the sleep laboratory.

hypnotic: An agent that promotes and aids sleep.

hypoglycemia: A condition in which the blood sugar drops to an abnormally low level.

hypomania: A mild mania.

hypothalamic-pituitary-adrenal (HPA) axis: A system that governs a multitude of hormonal activities in the body, including the body’s responses to stress.

hypothalamus: A network of nerves above the brainstem that regulates the body’s self-maintenance functions (such as blood pressure, temperature, and fluids). It receives signals from elsewhere in the body and secretes releases thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to produce thyroid-stimulating hormone (TSH). This network of communication between the hypothalamus, pituitary, and thyroid glands is referred to as the hypothalamic-pituitary axis (HP axis). It helps control the sympathetic and parasympathetic nervous systems and, through the pituitary gland, the endocrine system. Sparks the stress response by releasing the first of several chemical messengers that put the body on alert.

hypothyroidism: A disease that occurs when the thyroid fails to produce enough thyroid hormone to meet your body’s needs, thereby slowing your metabolism.

hysterectomy: Surgical removal of the uterus.