Illustration of the functioning of the thyroid gland. The thyroid is a gland that produces, from iodine, T3 and T4 hormones, which stimulate cell activity. The hypothalamus releases TRH, which stimulates the secretion of TSH by the pituitary gland, which in turn stimulates production of T3 and T4 hormones by the thyroid. These hormones in turn control the hypothalamus and pituitary glands and therefore the secretion of TRH and TSH. This mecanism allows a very delicate regulation of the level of thyroid hormones in the blood.