Regulates metabolic rate, growth, oxygen consumption, and brain function. Produces thyroxine (T4), triiodothyronine (T3), Calcitonin. Needs iodine to perform the synthesis of the hormones. TRH from hypothalamus -> TSH from anterior pituitary -> Thyroid hormones from gland. The increase of thyroid production turns TRH excretion from the hypothalamus.