thyroid gland, Endocrine gland in the throat that secretes hormones vital to metabolism and growth. Secretion of thyroid hormones—mostly thyroxine (T4)—is controlled by thyroid-stimulating hormone (TSH), released by the pituitary gland when the level of thyroid hormones in the blood drops below a certain threshold (see endocrine system). These hormones’ primary action in adults is to regulate cellular oxygen consumption (metabolic rate). They also lower blood cholesterol and are necessary for normal growth and development in children. The thyroid also produces calcitonin, a hormone that stimulates deposition of calcium from the blood into the bones, balancing the action of parathyroid hormone. See also goitre; Graves disease; iodine deficiency.
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