Triiodothyronine radioimmunoassay and its application to thyroid disorder

An accurate, highly sensitive triiodothyronine (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.