Toxic substances such as phenols, cadmium, mercury, etc, and medications such as propranolol, amiodarone and several others may interfere by stimulating or inhibiting the T4 to T3 conversion

· Feyes D, Hennemann G and Visser TJ. Inhibition of iodothyronine deiodinase by phenolphtalein dyes. Fed Eur Biomed Sci. 1982; 137(1):40-4

· Bahn AK, Mills JL, Snyder PJ, Gann PH, Houten L, Bialik O, Hollmann L, and Utiger RD. Hypothyroidism in workers exposed to polybrominated biphenyls. N Engl J Med. 1980; 302: 31-3

· Ikeda T, Ito Y, Murakami I, Mokuda O, Tominaga M and Mashiba H. Conversion of T4 to T3 in perfused liver of rats with carbontetrachloride-induced liver injury. Acta Endocrinol. 1986;112: 89-92

· Paier B, Hagmüller K, Nolli Mi, Gonzalez Pondal M, Stiegler C and Zaninovich AA. Changes induced by cadmium administration on thyroxine deiodination an

· Barregärd L, Lindstedt G, Schütz A, Sällsten G. Endocrine function in mercury exposed chloralkali workers. Occup Envir Med. 1994; 51: 536-40

Deficiencies in hormones (T3 itself, TSH, growth hormone, insulin, melatonin, etc) and trace elements (selenium, iron, zinc, cupper, etc) partially block this essential step for thyroid function

· Burger AG, Lambert M, Cullen M. Interférence de substances médicamenteuses dans la conversion de T4 en T3 et rT3 chez l’homme. Ann Endocrinol (Paris). 1981,42:461-9

· Grussendorf M, Hüfner M. Induction of the thyroxine to triiodothyronine converting enzyme in rat liver by thyroid hormones and analogs. Clin Chim Acta. 1977;80:61-6

· Erickson VJ, Cavalieri RR, Rosenberg LL. Thyroxine-5’-diodinase of rat thyroid, but not that of liver, is dependent on thyrotropin. Endocrinology. 1982;111:434-40

· Rezvani I, DiGeorge AM, Dowshen SA, Bourdony CJ. Action of human growth hormone on extrathyroidal conversion of thyroxine to triiodothyronine in children with hypopituitarism. Pediatr Res. 1981;15:6-9

· Schröder-Van der elst JP, Van der heide D. Effects of streptozocin-induced diabetes and food restriction on quantities and source of T4 and T3 in rat tissues. Diabetes. 1992;41:147-52

· Gavin LA, Mahon FA, Moeller M. The mechanism of impaired T3 production from T4 in diabetes. Diabetes. 1981;30:694-9

· Hoover PA, Vaughan MK, Little JC, Reiter RJ. N-methyl-D-aspartate does not prevent effects of melatonin on the reproductive and thyroid axes of male Syrian hamsters. J Endocrinology. 1992;133:51-8

· Chanoine J-P, Safran M, Farwell AP, Tranter P, Ekenbarger DM, Dubord S, Alex s, Arthur JR, Beckett GJ, Braverman LE, Leonard JL. Selenium deficiency and type II 5’-deiodinase regulation in the euthyroid and hypothyroid rat: evidence of a direct effect of thyroxine. Endocrinology. 1992;130:479-84

· Arthur JR, Nicol F, Beckett GJ. Selenium deficiency, thyroid hormone metabolism, and thyroid hormone deiodinases. Am J Clin Nutr Suppl. 1993; 57:236S-9S

· Beard J, Tobin B, and Green W. Evidence for thyroid hormone deficiency in iron-deficient anemic rats. J Nutr. 1989;772-8

· Fujimoto S, Indo Y, Higashi A, Matsuda I, Kashiwabara N, and Nakashima I. Conversion of thyroxine into triiodothyronine in zinc deficient rat liver. J Pediatr Gastroenterol Nutr. 1986;5:799-805

· Olin KI, Walter RM, and Keen CL. Copper deficiency affects selenoglutathione peroxidase and selenodeiodinase activities and antioxidant defense in weanling rats. Am J Clin Nutr 1994;59:654-8

On the other hand, excesses in hormones (glucocorticoids, ACTH, estrogens,…) and trace elements (iodine, lithium, …) may slow down this conversion

· Westgren U, Ahren B, Burger A, Ingemansson S, Melander A. Effects of dexamethasone, desoxycorticosterone, and ACTH on serum concentrations ot thyroxine, 3,5,3’-triiodothyronine and 3,3’,5’-triiodothyronine. Acta Med Scand. 1977;202 (1-2): 89-92

· Heyma P, Larkins RG. Glucocorticoids decrease the conversion of thyroxine into 3,5,3’-triiodothyronine by isolated rat renal tubules. Clin Science. 1982; 62: 215-20

· Scammell JG, Shiverick KT, Fregly MJ. Effect of chronic treatment with estrogen and thyroxine, alone and combined, on the rate of deiodination of l-thyroxine to 3,5,3’-triiodothyronine in vitro. Pharmacology. 1986;33: 52-7

· Aizawa T, Yamada T. Effects of thyroid hormones, antithyroid drugs and iodide on in vitro conversion of thyroxine to triiodothyronine. Clin Exp Pharmacol Physiol. 1981; 8: 215-25

· Voss C, Schrober HC, Hartmann N. Einfluss von Lithium auf die in vitro-Deioderung von l-Thyroxin in der Ratten leber. Acta Biol Med Germ. 1977; 36:1061-5

The absorption of oral T4 can be variable (50 to 73%), contrasting with that of T3 that is more constant and efficient (95%)

· Hays MT. Absorption of oral thyroxine in man. J Clin Endocrinol Metab. 1968; 28 (6):749-56

· Surks MI, Schodlow AR, Stock Jm, Oppenheimer JH. Determination of iodothyronine absorption and conversion of L-thyroxine using turnover rate techniques. J Clin Invest. 1973; 52:809-11

· Hays MT. Absorption of triidothyronine in man. J Clin Endocrinol Metab. 1970; 30(5):675-6