Serum triiodothyronine sulfate in man measured by radioimmunoassay

In humans deiodination and perhaps glucuronidation are important pathways of thyroid hormone metabolism. In animals, sulfation plays an important role in T4 and especially in T3 metabolism, but little is known about sulfate conjugation of thyroid hormone in humans. In this study we used a specific T3 sulfate (T3S) RIA to address this question. Eight normal subjects were given oral T3 (1 microgram/day.kg BW) for 7 weeks. During the fifth week they also received propylthiouracil (PTU; four doses of 250 mg/day) for 2 days and during the seventh week iopanoic acid (IOP; 1 g/day) for 3 days. The mean pre-T3 serum iodothyronine values were: T4, 92 +/- 6 (+/- SE) nmol/L; rT3, 0.24 +/- 0.02 nmol/L; T3, 2.30 +/- 0.10 nmol/L; and T3S, less than 0.1 nmol/L (at or below the detection limit of the RIA). After 4 weeks of T3 administration the mean serum values were: T4, 39 +/- 6; rT3, 0.11 +/- 0.01; T3, 5.31 +/- 0.39; and T3S, 0.10 +/- 0.01 nmol/L. After 2 days of PTU administration, mean serum T4 increased to 48 +/- 7 (P less than 0.005), rT3 to 0.20 +/- 0.03 (P less than 0.025), and T3S to 0.13 +/- 0.01 nmol/L (P = NS), but serum T3 did not change (4.91 +/- 0.35 nmol/L). The effect of IOP was more pronounced; after its administration for 3 days the mean serum T4 was 49 +/- 8 (P less than 0.001), rT3 was 0.48 +/- 0.09 (P less than 0.005), and T3S was 0.29 +/- 0.04 nmol/L (P less than 0.005), and serum T3 decreased to 3.95 +/- 0.25 nmol/L (P less than 0.005). The T3S/T3 ratio was increased by PTU from 0.018 +/- 0.003 to 0.024 +/- 0.004 (P less than = NS) and by IOP to 0.055 +/- 0.007 (P less than 0.005). In conclusion, 1) serum T3S is virtually undetectable (less than 0.1 nmol/L) in normal subjects; 2) low serum T3S concentrations are detected in humans given T3; 3) serum T3S in T3-treated subjects is increased by inhibition of type I deiodinase activity with PTU and especially IOP; and 4) in comparison with previous estimates of the serum T3S/T3 ratio in rats, the low ratio in humans may indicate that sulfation is not an important mechanism of T3 metabolism in humans and/or the kinetics of plasma T3 and T3S differ in humans and rats.