THE DEVELOPMENT OF A RADIOIMMUNOASSAY FOR REVERSE TRIIODOTHYRONINE SULFATE IN HUMAN SERUM AND AMNIOTIC-FLUID

Sulfated iodothyronines including T4-sulfate (T4S) and T3-sulfate (T3S ) have been identified in human serum and amniotic fluid. Little is kn own, however, about the existence of sulfate conjugation of reverse T3 (rT3S) in man. In this report, we employed a novel, sensitive, and sp ecific rT3S RIA to address this question. The rabbit antiserum to rT3S was highly specific; T4, T3, rT3, and 3,3'-T2 showed less than 0.002% cross-reaction with the antiserum. Only T4S and T3S cross-reacted sig nificantly (0.3% and 0.01%, respectively); other analogs cross-reacted less than 0.0001%. The detection threshold of the RIA was 14 pmol/L ( 1.0 ng/dL). The mean serum rT3S concentration (pmol/L) was 40 in euthy roid subjects. Values were similar in hypothyroid patients (38) and pr egnant women (52) but significantly (P < 0.01) elevated to 176 in hype rthyroid patient, 74 in patients with nonthyroid illnesses, and 684 in cord sera of newborns. Serum rT3S increased significantly in hyperthy roid patients 1 day after administration of 1 g sodium ipodate orally. Reverse T3S was detected consistently in amniotic fluid at 14 to 22 w eeks of gestation and showed a marked rise 1-3 weeks after intraamniot ic administration of 500-1000 mug T4. The various data suggest that: ( 1) rT3S is a normal component of human serum and amniotic fluid; (2) i t is derived from metabolism of T4 or rT3; (3) circulating rT3S increa ses in hyperthyroidism and in circumstances where type I 5'-monodeiodi nating activity is low, e.g. nonthyroid illnesses, fetal life, and aft er administration of ipodate.