thyroid hormone A chemical substance that is made by the thyroid gland and is essential for the function of every cell in the body. The thyroid gland uses iodine to make thyroid hormones, which help regulate growth and the rate of chemical reactions (metabolism) and are involved in the circadian rhythms that govern sleep, among other essential functions. The two most prominent thyroid hormones are thyroxine (T4) and triiodothyronine (T3). The thyroid gland also makes calcitonin, which is involved in calcium metabolization and bone strength, as well as other substances. Thyroid stimulating hormone (TSH), which is produced by the pituitary gland, acts to stimulate hormone production by the thyroid gland. The hypothalamus gland in the brain stimulates the pituitary gland to make TSH. See also calcitonin; thyroxine; triiodothyronine.

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