Urinary Excretion of Free and Conjugated 3′,5′-Diiodothyronine and 3,3′-Diiodothyronine*

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Abstract
Radioimmunoassays for the estimation of 3'',5''-diiodothyronine (3'',5''-T2) and 3,3''-diiodothyronine (3,3''-T2) in human urine were established. The urinary excretion of both glucuronide and sulfate conjugates of T2 and of T4 [thyroxine] T3 [triiodothyronine], and r[reverse]T3 were estimated by means of enzymatic deconjugation. In healthy controls, the mean excretion (pmol/24 h) of free T4 was 1820, that of free T3 was 813, that of free rT3 was 77, that of free 3'',5''-T2 was 13 and that of free 3,3''-T2 was 674. The total excretion of free and conjugated T4 was 2941, that of T3 was 1283, that of rT3 was 791, that of 3'',5''-T2 was 709 and that of 3,3''-T2 was 2688. Significant amounts of sulfated T4 and T3 were not demonstrated; the excretion of sulfated rT3 was higher than that of glucuronidated rT3 (P < 0.001). In contrast, glucuronidated and sulfated 3'',5''-T2 as well as glucuronidated and sulfated 3,3''-T2 were found in the urine in equal amounts. In hyperthyroidism, the excretions of free and glucuronidated iodothyronines were increased; the increase of the excretions of sulfated iodothyronines were less pronounced, only reaching statistical significance for 3,3''-T2 (P < 0.02). In hypothyroidism, the excretions of both free, glucuronidated and sulfated iodothyronines were reduced. Significant amounts of sulfated T4 and T3 could not be demonstrated in urine from hyperthyroid or hypothyroid patients. The amounts of free iodothyronines excreted in the urine vary considerably, suggesting active renal handling. The amounts of urinary glucuronidated and sulfated conjugates of the different iodothyronines studied vary considerably and are affected by thyroid function.