T cell: Abbreviation for T lymphocyte, a type of white blood cell that is produced in the bone marrow and is part of the body's immune system.

T lymphocyte: A type of white blood cell that is produced in the bone marrow and is part of the body's immune system. Also called a T cell.

tachycardia: An abnormally fast heartbeat, usually above 100 beats per minute.

tamoxifen: A drug used by women to prevent breast cancer or its recurrence.

tamsulosin: A drug used to treat symptoms of an enlarged prostate gland. It relieves constriction of the urethra and improves urine flow by relaxing the smooth muscle tissues in the capsule that surrounds the prostate. Unlike other drugs in this class, tamsulosin does not decrease blood pressure.

tangles: Also called neurofibrillary tangles—twisted strands of proteins that are found inside the dead or dying nerve cells of people with Alzheimer's disease.

tardive dyskinesia: Involuntary writhing movements of the arms, legs, and tongue caused by high doses of antipsychotic drugs over long periods of time.

tarsal coalition: An inherited condition in which two bones of the foot are fused together; can result in rigid flat feet.

tartar: A hardened layer of plaque that builds up on teeth. Also called calculus.

TDD: Abbreviation for telecommunications device for the deaf—machinery that allows a person who is deaf or hard of hearing to send and receive typed messages over the telephone.

telecommunications devices for the deaf: Machinery that allows a person who is deaf or hard of hearing to send and receive typed messages over the telephone.

tempo: When applied to exercise, the count for key movements in an exercise.

temporal arteritis: Also called cranial arteritis and giant cell arteritis—inflammation and damage to blood vessels supplying blood to the head and neck.

temporal lobe: One of the four major subdivisions of the two hemispheres of the brain's cerebral cortex. The temporal lobe plays a role in hearing, long-term memory, and behavior.

tendinitis: Inflammation of a tendon, usually caused by injury; may cause pain and restrict movement of the muscle attached to the tendon.

tendon: A cord of collagen fibers that connect a muscle to a bone.

tendonitis: Alternative spelling of tendinitis—inflammation of a tendon, usually caused by injury; may cause pain and restrict movement of the muscle attached to the tendon.

tenosynovitis: Swelling and inflammation of the protective sheath covering the tendons, which decreases the sheath's production of synovial fluid.

TENS: Abbreviation for transcutaneous electrical nerve stimulation—the use of low-voltage electrical current (through electrodes placed on the skin) to provide pain-suppressing stimulation.

tension headache: A headache, usually mild or moderate in intensity, not accompanied by other symptoms; pain is usually felt throughout the head, across the forehead, or in the back of the head. Also known as a muscle-contraction headache.

terazosin: A drug used to treat symptoms of an enlarged prostate gland. It relieves constriction of the urethra and improves urine flow by relaxing the smooth muscle tissue in the capsule that surrounds the prostate.

testosterone: A male hormone that stimulates bone and muscle growth and sexual development in men; also produced in lesser amounts in women, promoting sex drive and muscle growth.

thalamus: A brain structure that relays sensory information to other parts of the brain; also plays a role in memory consolidation.

theta waves: A pattern of brain waves on an electroencephalogram (EEG) characteristic of light, stage 1 sleep.

thiazolidinediones: A class of oral medication that improves sensitivity to insulin.

thoracic: Pertaining to the chest.

thrombolysis: Breaking up a blood clot.

thrombolytic agents: Agents or medications that dissolve blood clots and restore blood flow through a blocked artery; used to treat myocardial infarction, ischemic stroke, and pulmonary embolism. Also called clot busters. Examples include tissue plasminogen activator (tPA) and streptokinase.

thrombosis: Formation of a blood clot (called a thrombus) in a blood vessel or chamber of the heart.

thrombus: A blood clot that forms inside a blood vessel or chamber of the heart.

thunderclap headache: A sudden, excruciating headache that may be the result of bleeding in the head.

thymus: A specialized organ of the immune system located in the upper-middle chest where T cells mature.

thyroid gland: A two-lobed gland located in the front of the neck below the larynx (voice box). It secretes hormones that help regulate the body's metabolism and calcium balance.

thyroid hormone: Two iodine-containing hormones called thyroxine (T4) and triiodothyronine (T3). They help regulate the body's metabolism and calcium balance.

thyroidectomy: A surgical procedure to remove all or part of the thyroid.

thyroiditis: Inflammation of the thyroid gland. Types of thyroiditis include Hashimoto's thyroiditis, subacute thyroiditis, and postpartum or silent thyroiditis.

thyroid-stimulating hormone: A hormone secreted into the bloodstream by the pituitary gland that stimulates the thyroid gland. Doctors measure levels of TSH to determine whether a person's thyroid hormone levels are normal.

thyrotoxicosis: The presence of too much thyroid hormone in the body. This may be caused by an overproductive thyroid, inflammation of the thyroid, or taking too much thyroid hormone.

thyroxine: One of two types of major thyroid hormone manufactured by the thyroid gland. It contains four iodine atoms. Also known as T4.

TIA: Abbreviation for transient ischemic attack, a brain attack that resolves on its own within 24 hours. Sometimes called a mini-stroke, a TIA is often an early warning sign of an impending stroke.

tibia: The large bone of the calf, or shinbone.

tic douloureux: Pain from a disorder of the trigeminal nerve, the chief sensory nerve of the face. Also called trigeminal neuralgia.

ticlopidine: An antiplatelet drug that prevents the formation of blood clots.

tincture: An herbal product made by soaking an herb or other plant material in a mixture of water and alcohol to extract certain ingredients believed to be medicinal or beneficial.

tinea capitis: An itchy condition of the scalp caused by a fungal infection. Also known as ringworm.

tinea pedis: Athlete's foot.

tinnitus: A ringing in the ears or some other sound that has no external cause.

tissue: A group of cells that are specialized to do a certain job and are joined together to form a body structure, such as muscle or kidney.

tissue plasminogen activator: A clot-dissolving enzyme produced naturally in the blood vessels and artificially produced as a medication. Tissue plasminogen activator (commonly known as tPA) is used to break down blood clots in the treatment of heart attack, ischemic stroke, and pulmonary embolism. tPA must be used within a few hours after symptoms begin.

tolerable upper intake level: The highest amount of a nutrient deemed likely to have no harmful health effects for almost all healthy people when taken consistently.

tolerance: The process through which the body becomes less responsive to a psychoactive substance or rewarding behavior. Over time, people who develop tolerance need larger doses to get the same effect they first got with smaller doses.

toll-like receptors: One class of pattern-recognition receptors, found on the surfaces of the cells of the innate immune system.

tonic: An agent believed to invigorate a specific body organ.

tonometry: A glaucoma screening test that measures pressure inside the eye.

tooth decay: Infectious disease that attacks the teeth. Also called dental caries.

topical: Pertaining to an external surface of the body, such as the skin, mouth, vagina, or anus; often used to describe the administration of medicine that is applied directly to such a surface.

toxic: Pertaining to something that is poisonous.

toxic nodular goiter: An enlarged thyroid gland with nodules that produce excess thyroid hormone. This type of goiter is to blame for hyperthyroidism in many people over 60.

toxin: A poison, usually one produced by a living organism.

toxoid vaccines: Vaccines that protect against harmful bacterial toxins. These vaccines contain toxins that have been detoxified and rendered inactive.

tPA: Abbreviation for tissue plasminogen activator, a clot-dissolving enzyme produced naturally in the blood vessels and artificially produced as a medication. tPA is used to break down blood clots in the treatment of heart attack, ischemic stroke, and pulmonary embolism. tPA must be used within a few hours after symptoms begin.

trabecular bone: Bone tissue arranged in a meshwork of thin plates or beams that is commonly found at the center of long bones and that composes a large part of the hip and vertebrae. Also called cancellous bone or spongy bone.

trabecular meshwork: A system of fine, mesh-like tissue in the anterior chamber of the eye through which aqueous humor drains; located in the angle where the clear cornea, white sclera, and colored iris join.

trabeculectomy: A standard surgical procedure for glaucoma that creates a new channel for fluid drainage from the anterior chamber to the sub-conjunctival space.

trabeculoplasty: A laser procedure that burns small holes on the eye's trabecular meshwork to ease the flow of aqueous humor from the eye.

trace mineral: A mineral that is required only in tiny amounts in the diet to maintain health; the principal trace minerals are chromium, copper, selenium, sulfur, and zinc.

tracheostomy: A hole created through the front of the neck and into the windpipe (trachea). It provides an air passage when the usual route for breathing is obstructed, such as after a traumatic injury to the face or neck, or when long-term use of a breathing machine (ventilator) is needed.

tracheotomy: The procedure used to create a tracheostomy.

traction: The process of putting a bone or other body part under a pulling tension by applying weights and pulleys to help healing.

trans fatty acid: A type of fat made during hydrogenation of liquid vegetable oil. Trans fats are found in many solid margarines, commercially prepared baked goods, and fried foods in many restaurants. Trans fats increase harmful low-density lipoprotein, decrease protective high-density lipoprotein, and promote blood clotting and inflammation. Also known as trans fat.

transcranial Doppler scanning: An ultrasound technique that makes images of the major arteries at the base of the brain.

transcutaneous electrical nerve stimulation: The use of low-voltage electrical current (through electrodes placed on the skin) to provide pain-suppressing stimulation.

transdermal: Through the skin.

transferrin saturation: A measure of iron circulating in the blood.

transfusion: The process of taking blood from a healthy person and infusing it into a person whose own blood has been depleted during surgery or an accident, or is for some reason not functioning correctly. Transfusions of whole blood or of specific blood cells (such as red cells, white cells, or platelets) are possible.

transient ischemic attack: A brain attack that resolves on its own within 24 hours. Sometimes called a mini-stroke or TIA, a transient ischemic attack is often an early warning sign of an impending stroke.

transient pain: Minor, fleeting pain.

transmural infarction: Heart attack that destroys the entire thickness of a section of heart muscle.

transplantation: The process of removing an organ or other donated body part from one person and implanting it in another person.

transrectal ultrasonography: A procedure that uses sound waves to create an image of the prostate gland as a means of detecting cancer. Sound waves are directed to the prostate from a probe inserted in the rectum.

transurethral incision of the prostate: An operation used to treat benign prostatic hyperplasia (enlarged prostate gland) in which incisions are made in prostate tissue to relieve pressure on the urethra and ease urinary difficulties.

transurethral microwave thermotherapy: A heat therapy for benign prostatic hyperplasia (enlarged prostate gland) that uses microwaves to destroy prostate tissue that obstructs urine flow.

transurethral needle ablation: A procedure that uses radio waves to heat and destroy cells in the prostate gland that are obstructing the urethra.

transurethral resection of the prostate: An operation used to treat benign prostatic hyperplasia (enlarged prostate gland) in which excess prostate tissue is surgically removed.

transverse processes: The ringlike projection on each side of a vertebra to which muscles and ligaments are attached and, in the chest area, to which the ribs are connected.

traumatic grief: A prolonged, intense reaction to bereavement that affects one in 10 people who lose a loved one. Key signs are inability to accept the death; frequent nightmares and intrusive, upsetting memories; detachment from others; constant yearning for the deceased; and excessive loneliness. Also known as complicated grief or chronic grief.

tremor: A rhythmic, quivering movement of muscles that can be caused by diseases such as Parkinson disease, side effects of medication, or old age.

tricuspid valve: A three-flap valve that sits between the right atrium and the right ventricle.

tricyclic antidepressant: A class of medications that relieve depression by interfering with the reuptake of serotonin and norepinephrine by neurons in the brain and spinal cord. At low doses, they are effective as pain medicines.

trigeminal neuralgia: Pain from a disorder of the trigeminal nerve, the chief sensory nerve of the face. Also called tic douloureux.

trigger: Anything that can set off symptoms.

trigger point: A tender area that, when stimulated, also elicits pain elsewhere in the body.

triglyceride: The primary type of fat in the body and in the diet, formed from three fatty-acid molecules and one glycerol molecule. This fat can raise the risk for heart disease when elevated.

triiodothyronine: One of two types of major thyroid hormone manufactured by the thyroid gland. It contains three iodine atoms. Also known as T3.

triptans: A class of medications that work by constricting blood vessels in the head and perhaps by inhibiting inflammation.

trochlea: A groove in front of the femur where the patella moves as the knee bends and straightens.

troponins: Proteins found in heart muscle that leak into the circulation during a heart attack or other heart injury.

trust: A legal entity in which assets are gathered during a person's lifetime. That person may control distributions directly or through trustees elected to carry out wishes at a time or point specified. After death, remaining assets are distributed to beneficiaries.

trypsin: An enzyme secreted by the pancreas that helps digest proteins.

TSH: Abbreviation for thyroid-stimulating hormone, a hormone secreted into the bloodstream by the pituitary gland that stimulates the thyroid gland. Doctors measure levels of TSH to determine whether a person's thyroid hormone levels are normal.

tumor: Any type of swelling or enlargement of tissues; most often used to describe an abnormal growth of tissue, which can be cancerous or noncancerous.

tunica albuginea: The dense fibrous membrane surrounding each corpus cavernosum and the corpus spongiosum in the penis.

tympanic membrane: The eardrum.

tympanometry: A test of the eardrum's motion and pressure in the middle ear. Also known as impedance testing.

type 1 diabetes: A type of diabetes, a chronic condition that affects the way the body's tissues use sugar (glucose), their main source of fuel. Once called juvenile-onset diabetes and insulin-dependent diabetes. People with this form of diabetes don't make enough insulin, a hormone that controls the movement of glucose into cells. They must rely on insulin injections.

type 1 osteoporosis: Bone loss due to estrogen decline associated with menopause.

type 2 diabetes: A type of diabetes, a chronic condition that affects the way the body's tissues use sugar (glucose), their main source of fuel. Once called adult -onset diabetes and non-insulin-dependent diabetes. People with this form of the disease have tissues that either resist the effects of insulin (a hormone that control the movement of sugar into cells) or the body doesn't produce enough insulin. It is initially treated with diet, exercise, weight-loss if needed, and oral medications.

type 2 osteoporosis: Bone loss due to aging.