An improved radioimmunoassay of triiodothyronine in serum: its application to clinical and physiological studies.

Abstract

Abstract A sensitive, specific, convenient, and reproducible radioimmunoassay (RIA) for measurement of triiodothyronine (T 3 ) in unextracted serum is described. The features of the RIA which permit testing of unextracted serum include the use of a thyroidectomized sheep serum for construction of the standard curve and of 8-anilino-1-naphthalene sulfonic acid (ANS) to inhibit binding of T 3 to thyroxinebinding globulin (TBG). The serum T 3 concentration in 96 euthyroid subjects was 112.8 ± 3.3 ng. per 100 ml. (mean ± S.E.M.); it was 490.7 ± 42.3 ng. per 100 ml. in 30 hyperthyroid patients, 40.1 ± 7.6 ng. per 100 ml. in 12 hypothyroid patients, and 157.6 ± 31.2 ng. per 100 ml. in 10 euthyroid subjects with elevated serum TBG. Four normal subjects responded to intramuscular administration of TSH (Thytropar, 10 I.U.) with a maximal increase in serum T 3 of 75 to 195 per cent (mean 130) at 4 to 6 hours; the maximal increase in serum T 4 in these patients ranged between 36.4 and 92.7 per cent (mean 57.2), and occurred 12 to 24 hours after TSH administration. Seven euthyroid subjects, given 200 μg of thyrotropin-releasing hormone (TRH) intravenously, demonstrated a maximal increase in serum T 3 of 29 to 115 per cent (mean 72) at 2 to 4 hours; the maximal increase in serum T 4 also occurred at 2 to 4 hours of TRH administration but was more modest, 13 to 75 per cent (mean 37).