3,3‘ Diiodothyronine: Serum Concentration and Production in Normal and Abnormal Thyroid States

Summary: 3,3‘ diiodothyronine: serum concentration and production in normal and abnormal thyroid states. J. M. Corcoran and C. J. Eastrnan, Aust. N.Z. J. Med., 1980, 10, pp. 32–38. This study reports the details of a specific, sensitive radioimmunoassay for 3,3′ diiodothyronine (3,3'T2) in human serum and the results obtained from measurement of serum 3,3'T2 concentrations in normal humans and in patients with a variety of thyroidal and non thyroidal diseases. This assay permits measurement of 3,3'T2 in unextracted serum with a sensitivity of 10 to 20 pmolll and intraassay and interassay coefficients of variation of 3% and 9% respectively. Normal serum concentrations of 3,3'T2 in healthy, euthyroid adults ranged from 10 to 140 pmolll in males and from 8 to 196 pmolll in females. Mean levels were not significantly different. In women taking oral contraceptives serum 3,3'T2 concentrations ranged from 48 to 218 pmolll with a mean of 133 pmolll which was significantly higher than the normal female group (P <0.001): Mean serum 3,3'T2 concentrations were increased in hyperthyroid and decreased in hypothyroid patients, however, there was considerable overlap of individual results with normal subjects. Serum 3,3'T, levels in sick euthyroid patients were mostly below assay sensitivity. By contrast serum 3,3'T2 levels in cord blood were significantly higher than in normal adults with a range of 173 to 405 pmolll and mean of 287 pmolll. Serial studies performed on hypo‐thyroid patients given either L‐T3 (60 μg/day) or L‐reverse T3 (120 μg/day) in divided dosage revealed significant rises in serum 3,3'T2 levels. These results demonstrated extrathyroidal conversion of both T3 and reverse T3 to 3,3'T2 in peripheral tissues. Data obtained from kinetic studies of 3,3'T2 metabolism in normal adults revealed a mean metabolic clearance rate of 175 ± 112 I/day (range 110–276 I/day), and a mean half life of 8.3±1.0 hours. These data are consistent with an average daily production rate of 15.75 nmollday. It is concluded that 3,3'T2 is a normal constituent of human serum and is derived from peripheral mono deiodination of both T3 and reverse T3. The biological significance of this metabolite has not yet been determined.