Iodine enters the thyroid follicular cells as inorganic iodide and is incorporated through a series of metabolic steps into the main thyroid hormones thyroxine (T4 ) and 3,3',5-triiodothyronine (T3 ). Consecutive steps in this metabolic sequence are: 1) active transport of iodide by the thyroid follicular cell; 2) iodination of tyrosyl residues of thyroglobulin at the apical membrane; 3) coupling of mono- and diiodotyrosine molecules within thyroglobulin to form T4 and T 3 ; 4) proteolysis of thyroglobulin, with release of T4, T3 and iodotyrosines; and 5) deiodination of iodotyrosines within the thyroid and reutilization of the liberated iodide. The iodothyronines are then secreted into the blood. The production and secretion of iodothyronines by the thyroid are under control of thyroid stimulating hormone (TSH) secreted by the anterior pituitary gland. TSH binds to a specific plasma membrane receptor on the surface of the follicular cells and exerts its action mainly via the second messenger cAMP (1). The secretion of TSH is regulated by two interacting elements: neural control by the hypothalamus (stimulation by TRH, inhibition by somatostatin (2) and dopamine (3)) and negative feedback control by thyroid hormones (4-6). The main product secreted by the thyroid is T4 (structural formulas of the iodothyronines are shown in Fig. 1). In the normal adult the mean T4 secretion rate is -115 nmol/d per 70 kg body weight (BW). Thyroidal secretion of T 3 amounts to-g and of 3,3',5'-triiodothyronine (reverse T3 , rT 3) to -z nmol/d per 70 kg BW (for a review, see ref. 7). As the total production rates of T3 (-43 nmol/d) and rT3 (45-90 nmol/d) are much higher (7), it will be clear that the production of these iodothyronines occurs predominantly outside the thyroid. Contribution of thyroidal seretion to the circulating diiodothyronines 3,3'-T2 , 3,5-T2 and 3',5'-T2 is negligible (8,9).

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Erasmus University Rotterdam
T.J. Visser (Theo)
hdl.handle.net/1765/51055
Erasmus MC: University Medical Center Rotterdam

Eelkman Rooda, S. J. (1988, November 16). Deiodination and conjugation of T3 and other iodothyronines. Retrieved from http://hdl.handle.net/1765/51055