thyroid hormone

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thyroid hormone n. an iodine-containing substance, synthesized and secreted by the thyroid gland, that acts to increase the basal metabolic rate, being essential for normal mental and physical development. The principal thyroid hormone, thyroxine (tetraiodothyronine, T4), is mostly converted to the metabolically more active form, triiodothyronine (T3), within the peripheral tissues. Lack of these hormones gives rise to cretinism in infants and myxoedema in adults. Excessive production of thyroid hormones gives rise to thyrotoxicosis.