Direct immunoassay of triiodothyronine in human serum

Open Access
Publisher Website
Google Scholar
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. 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.