Alzheimer's disease: A severe, degenerative, incurable disease that affects the brain and the ability to remember, think, and function. See our other Hot Topics: Alzheimer's Disease: A Family Affair and a Growing Social Problem, and Alzheimer's Disease.

Antibodies: Substances produced by the immune system that attack substances the immune system recognizes as foreign, and not part of the self. Antibodies attach to the foreign substance and alert the immune system to destroy it.

Antidepressants: Drugs used to treat clinical depression. They can affect T3 metabolism in the brain.

Autoimmune disease: Disease that occurs when the body's immune system accidentally recognizes a natural substance produced by the body as foreign, for unknown reasons, and attacks that substance. Some autoimmune diseases are insulin-dependent diabetes mellitus, lupus, thyroid disease, and vitiligo (loss of skin color).

Cardiovascular fitness: The proper working of the heart and blood system and its ability to handle exercise by delivering enough blood, oxygen and fuel to the body.

Cholesterol: A waxy substance that the body uses to synthesize steroid hormones (sex hormones, coristol, etc.). When present in excessive quantities in the blood, it increases the risk of heart disease. Cholesterol can be reduced through exercise and careful diet if thyroid hormone levels are normal.

Chordates: Animals with spinal cords. This includes all vertebrates (animals with backbones) and a few other animals with spinal cords.

Cretinism: Mental retardation and physical problems caused by hypothyroidism from birth which was not treated soon enough with thyroid hormone replacements. The U.S. tests all infants at birth for hypothyroidism to prevent cretinism.

Depressants: Drugs that depress the brain, slow reflexes, numb sensation, etc. Depressants can affect T3 metabolism by slowing breakdown of T3, which leads to prolonged levels of T3 beyond appropriate brain function. This may affect mood.

Depression: A condition characterized by lack of interest in life, numbed emotions or extremely negative feelings, problems focusing, withdrawal from people and social life, etc. Depression without apparent cause that does not resolve by itself is a sign of a serious problem.

Estrogen: Known as the female hormone, it encourages growth of female reproductive tissue. It can cancel out the effects of another female hormone, progesterone, and of thyroid hormone. The more estrogen there is in the body, the more thyroid hormone is needed for proper function. Many plants (like soy and corn) have compounds that can mimic the effects of estrogen. Eating a diet high in these substances can increase the body's requirement for thyroid hormone.

Fatty acids: Substances created during the digestion of fat. They travel through the blood and are used for fuel or are stored by fat cells.

Genes: Stretches of DNA containing coded instructions to make proteins.

Gene transcription: The process of reading genes and making proteins according to their instructions; turning genes on.

Genome: The total set of all genes and other DNA in the nucleus of each cell.

Goiter: The term for an enlarged thyroid gland that is trying to make enough thyroid hormone to meet a hypothyroid body's needs. Goiters can range in size from small enough to be noticeable only through touch to bigger than a human baby.

Goitrogens: Substances found primarily in plants that impair the ability of the body to use iodine to synthesize thyroid hormone. When TH synthesis is affected by goitrogens, the body may grow a goiter in an effort to make enough thyroid hormone.

Graves' disease: Autoimmune hyperthyroidism caused by antibodies continually stimulating the thyroid to make more TH. Many therapies exist, including total destruction of the thyroid, followed by thyroid hormone replacement therapy for the remainder of the patient's life.

Hashimoto's thyroiditis: An autoimmune thyroid disease that causes goiter, hypothyroidism, and progressive destruction of the thyroid. It results in unstable thyroid hormone levels, which can be controlled by thyroid hormone replacement therapy. There is no known cause or other therapy.

Hormones: Substances released by one part of the body to influence processes in other parts of the body. They usually travel in the bloodstream. Insulin, estrogen, growth hormone, thyroid hormone, and TSH are all hormones.

Hypothyroidism (Hypo=low): The body's condition when thyroid hormone levels are insufficent for normal body function. Symptoms include slowed metabolism, fatigue, mental problems, hypoglycemia, problems breathing, slow heartbeat, and high cholesterol.

Hyperthyroidism (Hyper=high): The body's condition when thyroid hormone levels are excessive. Symptoms include high metabolism, rapid heart rate, fatigue, and manic-depressive behavior.

Iodine: An element (I) found in nature. It is more common in the ocean than on land, and is essential for making thyroid hormone. Iodine deficiency leads to hypothyroidism. A sudden excess in an iodine-deficient person can cause a brief bout of hyperthyroidism.

Ketone bodies: Byproducts of fatty acid breakdown. When present in excess over a long period of time, they can be damaging to the body. Ketone body concentrations increase during episodes of starvation or diabetes mellitus as the body runs low on glucose (sugar).

Levothyroxine (T4): The relatively inactive form of thyroid hormone, with four iodine atoms. It is converted by body tissues to T3 (triiodothyronine) when needed. It makes up 80% of the thyroid hormone released by the thyroid.

Manic-depression (bipolar disorder): A mood disorder characterized by grandiose feelings, accelerated motions and thoughts, rapid speech, and impulsive decisions. The "manic" phase inevitably falls back to a normal or very depressive state. Hyperthyroidism can be mistaken for manic-depression.

Mitochondria: Small organelles within each cell that have their own genes and act as powerhouses for the cell, producing ATP for the cell to use for energy and heat.

Neurotransmitters: Chemicals in the brain that help communicate messages from cell to cell, and are crucial in thinking, memory, and emotion.

Nuclear receptors: Proteins that are present in the nucleus and can bind to hormones. When they do so, they then act as transcription factors and turn on genes.

Proteins: The building blocks of the body, which are assembled according to genes.

Responsive elements: Short stretches of DNA, to which transcription factors bind in order to turn genes on or off.

Retinoid x receptors: Nuclear receptors that are activated by various retinoids. Retinoid X receptors often are dimerized with other nuclear receptors.

Serotonin: A neurotransmitter that is essential for a sense of well-being and good mood. Many anti-depressants work by lifting serotonin levels.

Stimulants: Substances that stimulate the body's systems, including heart rate, breathing, and metabolism. They can depress thyroid hormone levels while they are in the body.

Thyroid (from Greek, "shield"): A butterfly shaped gland behind the larynx that produces thyroid hormone. It is larger in women than men.

Thyroid hormone (TH): A hormone containing iodine that affects the body's metabolism. It exists in two forms, levothyroxine (T4) and triiodothyronine (T3).

Thyroid-stimulating hormone (TSH): The hormone released by the pituitary gland that stimulates the thyroid to grow and make more TH.

Thyroid hormone receptors (TRs): Proteins that become activated when they bind to TH and are then able to bind to responsive elements in the DNA. TRs turn genes on or off when they bind to the DNA. Mutations in the two genes that code for TRs can cause symptoms of hypothyroidism.

Triglycerides: Fat by-products. The presence of triglycerides in the blood is a risk factor for heart disease and indicates that the body is not metabolizing fat properly. Triglycerides can be reduced through exercise if thyroid hormone levels are normal.

Triiodothyronine (T3): The active form of thyroid hormone. It has three iodine atoms and is eight times more active than levothyroxine. It is the essential form used in the brain.

Vitamin A: Substances (retinol, dehydroretinol, retinoic acid) that work like retinol in the body and are essential to health and development. Vitamin A can be synthesized in the body from carotenes.