Identification of Thyroxine-Sulfate (T 4 S) in Human Serum and Amniotic Fluid by a Novel T 4 S Radioimmunoassay

Abstract

Recently, we identified significant amounts of thyroxine sulfate (T₄S) in fetal sheep serum, meconium, bile, and amniotic and allantoic fluids. Little is known, however, about sulfate conjugation of thyroxine in humans. In this study, we employed a novel, sensitive T₄S RIA to address this question. The rabbit antiserum was quite specific; T₄, T₃, rT₃, and 3,3′-T₂ showed less than 0.002% cross-reactivity. Other analogs cross-reacted less than 0.0001 %. Only rT₃S and T₃S cross-reacted significantly (9.9% and 2.0%, respectively). The mean serum T₄S concentration (ng/dL) was 8.6 in euthyroid subjects, 14.4 in hyperthyroid subjects, 5.0 in hypothyroid subjects, 5.9 in pregnancy, and 4.5 in patients with nonthyroid illnesses. T₄S concentration in amniotic fluid from women at 18-19 weeks of gestation (25.5 ng/dL) was higher than that at 14-15 weeks of gestation (14.3 ng/dL). A significant rise in serum T₄S was detected in hyperthyroid patients 1 day after ingestion of 1 g of ipodate. These data suggest that T₄S is a normal component of human serum and amniotic fluid, and it is mostly derived from T₄ peripherally and accumulates when type I 5′-monodeiodinating activity is low in fetuses or inhibited by drugs, such as ipodate.