The thyroid gland is an endocrine gland, found in almost all vertebrate animals, that synthesizes, stores, and secretes two hormones that affect metabolism and growth. The hormones are thyroxine, also called tetraiodothyronine (T₄), and triiodothyronine (T₃). These hormones are composed of the amino acid tyrosine, and contain four and three iodine atoms, respectively. The thyroid gland constitutes only about 0.5% of the total human body weight, but holds about 25% of the total iodine in the body, which is obtained from dietary food and water. Iodine circulates in the blood as inorganic iodide, but is concentrated in the thyroid up to 500 times the level in the blood and is essential for synthesis of the hormones.

The thyroid gland is located below the larynx and consists of two lobes connected by an isthmus (band of tissue). It is brownish-red and normally weighs about 1 oz (28 g). The gland is composed...