THYROID HORMONES The thyroid gland secretes 3 3 hormones-thyro-xine (T4), triiodothyronine (T,) and calcitonin. The former two are produced by thyroid follicles,have similar biological activity and the term 'thyroid hormone' is restricted to these only. Calcitonin produced by interfollicular "C cells is chemically and biologically entirely different. It is considered along with parathormone with which it regulates calcium metabolism. The physiological significance of thyroid gland was recognized only after Graves and Basedow (1835, 1840) associated the clinical features of the Graves' disease' with swelling of thyroid gland and Gull (1874) correlated myxoedema with its atrophy. Kendall (1915) obtained erystalline thyroxine and postulated its chemical formula which was confirmed in 1926. Thyroxine was the first hormone to be synthesized in the laboratory. Since T could not account for all the biological activity of thy- roid extract, search was made and...
