Visser T J
Department of Internal Medicine III, Erasmus University Medical School, Rotterdam, The Netherlands.
Acta Med Austriaca. 1996;23(1-2):10-6.
T4 is the main product secreted by the thyroid follicular cells and is regarded as a precursor of the bioactive hormone T3, most of which is produced by outer ring deiodination of T4 in peripheral tissues. Both T4 and T3 are inactivated by inner ring deiodination. Three deiodinases have been identified with outer and/or inner ring deiodinase activities, which play an important role in the tissue-specific regulation of thyroid hormone bioactivity. All three enzymes have recently been shown to contain selenocysteine residues. The second important pathway of thyroid hormone metabolism involves the conjugation of the phenolic hydroxyl group with sulfate or glucuronic acid. The glucuronides are excreted in bile, acting as intermediates in the enterohepatic cycle and fecal excretion of thyroid hormone. Sulfation accelerates the deiodination of different iodothyronines by the type I deiodinase and, thus, initiates the irreversible degradation of the hormone. If type I deiodinases activity is low, e.g. in the fetus, T3 sulfate may function as a reservoir from which active T3 is recovered by tissue sulfatase activity.
T4是甲状腺滤泡细胞分泌的主要产物，被视为生物活性激素T3的前体，其中大部分是由外周组织中T4的外环脱碘产生的。T4和T3都会通过内环脱碘而失活。已鉴定出三种具有外环和/或内环脱碘酶活性的脱碘酶，它们在甲状腺激素生物活性的组织特异性调节中起重要作用。最近发现这三种酶都含有硒代半胱氨酸残基。甲状腺激素代谢的第二个重要途径涉及酚羟基与硫酸盐或葡萄糖醛酸的结合。葡萄糖醛酸苷通过胆汁排出，作为甲状腺激素肠肝循环和粪便排泄的中间体。硫酸化通过I型脱碘酶加速不同碘甲状腺原氨酸的脱碘，从而启动激素的不可逆降解。如果I型脱碘酶活性较低，例如在胎儿中，硫酸T3可能作为一种储备，通过组织硫酸酯酶活性从中回收活性T3。
