any of the hormones synthesized and released by the thyroid gland. The primary thyroid hormone, thyroxine (T4), is metabolized to triiodothyronine (T3) within target tissues. Plasma levels of T4 are much higher than those of T3, but T3 has the more potent physiological activity. Both hormones play a central role in regulating basic metabolic processes and the early development of the brain, and their excess secretion has major effects on functioning (see congenital hypothyroidism; myxedema; thyrotoxicosis). Calcitonin, a hormone released by parafollicular cells of the thyroid gland, plays a crucial role in calcium and phosphate metabolism.