Radioimmunoassay of triiodothyronine in unextracted human serum

Serum triiodothyronine (T-3) concentrations have been estimated by radioimmunoassay using unextracted serum. The serum T-3 concentrations have been shown to be similar in two separate European populations (0.76-1.67 ng/ml). Raised T-3 values have been observed in all subjects with hyperthyroidism. Low values are seen in hypothyroidism although there is some overlap with the normal range. There is a good correlation between serum T-3 and serum thyroxine (T-4) concentrations, and estimation of T-3 seems likely to prove a practical and reliable test of thyroid function.