Thyroxine sulfate

Thyroxine sulfate is a thyroid hormone metabolite. In Vitro: Thyroxine sulfate (T4S) is a normal component of human serum and amniotic fluid, and it is mostly derived from thyroxine peripherally and accumulates when type I 5-monodeiodinating activity is low in fetuses or inhibited by drugs, such as ipodate^[1]. In Vivo: Significant amounts of thyroxine sulfate (T4S) in fetal sheep serum, meconium, bile, and amniotic and allantoic fluids are observed. T4S concentration in amniotic fluid from women at 18-19 weeks of gestation (25.5 ng/dL) is higher than that at 14-15 weeks of gestation (14.3 ng/dL). A significant rise in serum T4S is detected in hyperthyroid patients 1 day after ingestion of 1 g of ipodate^[1]. Thyroxine undergoes significant sulfation in rats, and biliary excretion of T4S is enhanced if its type I deiodination is inhibited^[2]. Serum T4S levels are clearly elevated compared with healthy references, and the decreased deiodination by liver D1 during critical illness appears to play a role in this increase in serum T4S levels^[3].