ABSTRACT
The thyroid secretes thyroxine (T4) and tri-iodothyronine (T3), as well as calcitonin, which is discussed in Chapter 39. The release of T3 and T4 is controlled by the pituitary hormone thyrotrophin (thyroid-stimulating hormone, TSH). This binds to receptors on thyroid follicular cells and activates adenylyl cyclase, which stimulates iodine trapping, iodothyronine synthesis and release of thyroid hormones. TSH is secreted by basophil cells in the adenohypophysis. Secretion of TSH by the anterior pituitary is stimulated by the hypothalamic peptide thyrotrophin-releasing hormone (TRH). Circulating T4 and T3 produce negative-feedback inhibition of TSH at the pituitary and hypothalamus.
