Introduction The thyroid produces hormones that contain iodine T4 is converted to active T3 and inactive reverse T3 (rT3) in peripheral tissue T3 is more potent than T4 rT3 is inactive the thyroid controls the body’s metabolic rate Physiology Functions of T3 Brain maturation Bone growth ↑ Beta-adrenergic ↑ Basal metabolic rate ↑ Blood glucose Breakdown lipids (↑ lipolysis) Synthesis synthesized in thyroid gland and stored in follicles thyroid peroxidase catalyzes oxidation, organification, and coupling T4 is converted to rT3 via 5'-deiodinase, which removes an iodine from the inner ring of T4 T4 is converted to T3 in peripheral tissues via 5’-deiodinase, which removes an iodine from the outer ring of T4 inhibited by steroids, beta-blockers, and propylthiouracil Regulation of thyroid hormones production of thyroid hormones thyrotropin-releasing hormone (TRH) release from the hypothalamus stimulates thyroid-stimulating hormone (TSH) release from the pituitary gland TSH then stimulates thyroid follicular cells to produce T3/T4 In cases of papillary thyroid carcinoma, patients are often treated with levothyroxine after thyroidectomy such that TSH levels are suppressed, in order to prevent TSH stimulation of any remaining malignant cells negative feedback loop free T3/T4 causes ↓ TRH secretion and ↓ sensitivity of pituitary to TRH bound T3/T4 are inactive thyroxine-binding globulin (TBG) binds to T3/T4, making them inactive