Thyroid Hormone

THYROID HORMONE

The thyroid gland secretes 3 hormones—thyroxine (T₄), triiodothyronine (T₃) 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, (Ch. No. 24) 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 crystalline thyroxine and suggested its chemical formula which was confirmed in 1926. Thyroxine was the first hormone to be synthesized in the laboratory. Later, as T₄ could not account for all the biological activity of thyroid extract, search was made and more potent T₃ was discovered in 1952.

Chemistry

Both T₄ and T₃ are iodine containing derivatives of thyronine which is a condensation product of two molecules of the amino acid tyrosine. Thyroxine; is 3, 5, 3´, 5´–tetraiodothyronine while T₃ is 3, 5, 3´ triiodothyronine.