A sensitive and precise radioimmunoassay for the direct measurement of triiodothyronine (T₃) in human serum has been designed using sodium salicylate to block T₃-TBG binding. This assay is sufficiently sensitive to quantitate T₃ accurately in 50-100 μl of normal serum and to measure quantities as small as 12.5 pg in 0.2 ml of hypothyroid serum. The T₃ values observed in euthyroid subjects and in patients with various thyroid diseases are as follows: euthyroid (38) 1.10±0.25 (SD) ng/ml, hypothyroid (25) 0.39±0.21 (SD) ng/ml, and hyperthyroid (24) 5.46±4.42 (SD) ng/ml. The levels of T₃ parallel the thyroxine (T₄) concentration in the sera of these subjects. In eight pregnant women at the time of delivery, T₃ concentrations were in the upper normal range (mean 1.33 ng/ml). The levels of T₃ in cord serum obtained at the time of delivery of these patients (mean 0.53 ng/ml) are distinctly lower and close to the hypothyroid mean.

Administration of 10 U of bovine thyroid-stimulating hormone (TSH) to euthyroid subjects causes a two-fold increase in serum T₃ levels within 8 hr. At this time, the increase in serum T₄ concentration is only 41%. In two subjects in whom thyroid secretion was acutely inhibited, one after pituitary surgery and another after thyroidectomy, the serum T₃ fell into the hypothyroid range within 1-2 days. Thus, serum T₃ concentrations appear to be a sensitive index of acute changes in thyroid hormone secretion and should prove to be a useful adjunct to both the clinical and physiological evaluation of thyroid function.