Triiodothyronine radioimmunoassay and its application to thyroid disorders.

An accurate, highly sensitive triiodothyrone (T3) radioimmunoassay system is described. Direct measurement in serum is made possible by the use of T3-free serum for standards, and blocking of T3 binding to thyroxine (T4)-binding globulin with salicylate. T4 cross reactivity was less than 0.3%. Mean T3 levels in 55 euthyroid, 32 hyperthyroid and 19 primary hypothyroid patients were 1.46 +/- 0.17, 5.34 +/- 1.86 and 0.49 +/- 0.34 (SD) ng/ml, respectively. T3 was found to be a more sensitive index than T4 in hyperthyroidism; in hypothyroidism it overlapped with euthyroidism. The presence of goiter had no effect on T3 concentrations in euthyroid patients. Among the patients studied, eight were found to have T3 toxicosis, including two patients with recurrent thyrotoxicosis. A significant T3 elevation was also found in euthyroid patients after thyroidectomy whereas T4 remained normal. The extent of T4 and T3 elevation was similar during the initial phase of subacute thyroiditis. There were significantly lower T3 values and higher thyroid-stimulating hormone values in umbilical cord blood than in maternal blood. In two euthyroid subjects, thyrotropin-releasing hormone stimulation caused a 50% rise in T3 levels and no change in T4 levels over a 2-h period.