Simultaneous measurement of 3,5-diiodothyronine and 3,5,3'-triiodothyronine turnover kinetics in euthyroid hyperthyroid, and hypothyroid subjects.

Simultaneous kinetic studies of 3,5-diiodothyronine (3,5-T2) and T3 were performed in 15 healthy controls (8 men and 7 women), 7 hyperthyroid patients (2 men and 5 women), and 6 hypothyroid women using the single injection, noncompartmental approach. The serum concentrations (picomoles per liter), MCRs (liters . day-1 . (70 kg)-1), and production rates (PRs; nmol . day-1 . (70 kg)-1) of 3,5-T2 in healthy men and women were (mean +/- SD): 100 +/- 23 vs. 80 +/- 23 (P = NS), 59 +/- 31 vs. 123 +/- 58 (P less than 0.025), and 5.6 +/- 1.9 vs. 9.1 +/- 2.6 (P less than 0.02). The conversion rate (CR) of T3 to 3,5-T2 was 12.0 +/- 3.8% in men compared to 18.5 +/- 3.7% in women (P less than 0.01). Serum 3,5-T2 levels in five mildly hyperthyroid women were elevated to 123 +/- 33 pmol/liter (P less than 0.05), whereas the MCR and PR were unchanged. However, two hyperthyroid men with more pronounced elevation of serum T3 had enhanced PRs (26.9 and 23.9 nmol . day-1 . (70 kg)-1). The CR in hyperthyroid women was significantly reduced to 5.6 +/- 2.9% (P less than 0.001). The serum levels, MCR, and PR of 3,5-T2 in hypothyroid women were: 58 +/- 25 pmol/liter (P = NS), 71 +/- 52 liters . day-1 . (70 kg)-1 (P = NS), and 3.4 +/- 2.4 nmol . day-1 . (70 kg)-1 (P less than 0.005). The CR was enhanced to 34.8 +/- 15.7% (P less than 0.05). Our data demonstrate that in euthyroid subjects, approximately 15% of T3 is deiodinated to 3,5-T2, and this 5'-deiodination of T3 is influenced by thyroid function.