APA Dictionary of Psychology

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.