Serum 3,3'-L-Diiodothyronine, a Direct Radioimmunoassay in Human Serum: Method and Clinical Results

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Abstract
A specific radioimmunoassay for 3,3'– diiodothyronine (T2) is described which is capable of detecting as little as 1.3 ng/dl. The antiserum recognizes mainly T2; biliary conjugates of T2 bind slightly to the antibody. Theintraassay and interassay coefficients of variation were, respectively, 5.7% and 13.1%. T2 was detected in the serum of hypothyroid patients treatedwith triiodothyronine (T3) and in euthyroid subjects treated with reverse triiodothyronine (rT3). These results suggest that both T3 and rT3 are deiodinated to T2. Serum concentrations of T2 in normal subjects decreased with age. Between 20 and 40 years the mean concentration was 4.3 ± 2.0 ng/dl (2 SD), between 41 and60 years it varied from 1.9 to 5.8 ng/dl (3.8 ± 0.3 ng/dl, SE) and in elderly subjects have 60 years concentrations varied from immeasurable to 4 ng/dl (2.9 ± 0.4 ng/dl, SE). Lowserum T2 concentrations were foundin anorexia nervosa (2.5 ± 0.3 ng/dl, SE). In hypothyroidism the serum concentrations were lowor immeasurable. As most of the hypothyroid subjects were elderly their serum T2 concentrations overlapped with the low values found in the elderly euthyroidsubjects. In classical hyperthyroidism serum T2 concentrations were greatly increased (3.3 to 31 ng/dl (11.8 ± 2.7 ng/dl, SE) but in “T3 toxicosis” theconcentrations were only modestly increased (2.4 to 8.8 ng/dl, 5.2 ± 0.8 ng/dl, SE).