It produces six different hormones. Prolactin causes the mammary glands to produce milk after childbirth. Thyroxin is usually secreted as T4, which is converted to T3, the active form. Iodine is a necessary component of thyroxin. Thyroxin increases metabolic rate and stimulates all organs to metabolize faster.
Simple goiter is caused by insufficient iodine in the diet.
TSH from the anterior pituitary overstimulates the thyroid, causing it to enlarge when iodine is absent. Cretinism results when the thyroid fails to develop normally. Hyperthyroidism is caused by an overactive thyroid in Graves' disease. The thyroid also secretes calcitonin, which promotes calcium absorption by bones. Parathyroid hormone causes the blood phosphate level to decrease and the blood calcium level to increase by demineralizing bones and increasing calcium retention by kidneys and intestines.