The thyroid produces thyroxin (also called T4 because it contains 4 iodine atoms) and triiodothyronine (also called T3 because it contains 3 iodine atoms).
Both T4 and T3 have similar effects on target cells. In most target tissues, T4 is converted to T3. They influence metabolic rate, growth, and development.
Thyroxin production is regulated by a negative feedback mechanism in which it inhibits the hypothalamus from stimulating the thyroid.
Hypothyroidism occurs when the thyroids produce too little hormone. In adults, it results in lethargy and weight gain. In infants, it causes cretinism, which is characterized by dwarfism, mental retardation, and lack of sexual maturity. Administering thyroid hormones treats these affects.
Too much T3 and T4 (hyperthyroidism) increases heart rate and blood pressure, and causes weight loss.
Iodine is needed to manufacture thyroid hormones. A deficiency in iodine prevents the synthesis of thyroid hormones which, in turn, results in an excess of thyroid stimulating hormone being produced by the anterior pituitary. A goiter results when constant stimulation of the thyroid causes it to enlarge.
The thyroid gland also secretes calcitonin, which stimulates calcium deposition in the bones. This is the opposite of the action of parathyroid hormone (see below).
Calcitonin production is not regulated by the anterior pituitary. It's secretion is stimulated by high calcium levels in the blood.
The pituitary contains two lobes. Hormones released by the posterior lobe are synthesized by neurons in the hypothalamus. Unlike the posterior lobe, the anterior lobe produces the hormones that it releases.
Refer to the diagram below as you read about the hypothalamus, pituitary, and each of the glands they control.
The thyroid gland is one of the body's largest endocrine glands. It is situated in the neck just below the larynx and is composed of two lobes, one on either side of the trachea. The thyroid gland produces two hormones, T3 (triiodothyronine) and T4 (thyroxine). The thyroid cells that synthesize T4 and T3 take up circulating iodine from the blood and attach them to tyrosine residues in the protein thyroglobulin. Two molecules of iodinated tyrosine combine to form thyroxine, which then combines with thyroglobulin, a polysaccharide-protein substance. In this bound form, the thyroid can store a supply of thyroxine lasting several weeks. These actions are stimulated by the binding of thyroid stimulating hormone (TSH) to transmembrane receptors at the cell surface. TSH also activates several rate-limiting steps that control the rate of release of thyroxine.
Both T3 and T4 are derivatives of the amino acid tyrosine. T4 contains four atoms of iodine, while T3 contains three. The thyroid secretes 80% T4 but when T4 enters target cells, one atom of iodine is removed, converting T4 into T3, which is more potent. The same receptors bind both T3 and T4 to the target cells. In the target cells, T3 increases metabolic rate (indicated by a rise in body temperature and in oxygen consumption, and increases the rate and strength of the heart beat). This change in metabolic rate does not affect the brain. Thyroxine is also required for normal growth, apparently by stimulating the effects of growth hormone on protein synthesis. A lack of thyroxine reduces the ability of GH to