Direct immunoassay of triiodothyronine in human serum

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Abstract

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.

Authors

P. R. Larsen