Radioimmunoassay of Thyrotropin in Human Serum

A radioimmunoassay capable of measuring thyrotropin (TSH) in human serum has been developed. Separation of antibody-bound from free 131I-TSH was accomplished by differential solubility in alcoholsaline. Partial cross-reactions of human TSH with human chorionic gonadotropin (HCG) and human luteinizing hormone (HLH) were demonstrated. In order to ensure specificity of the radioimmunoassay, the antisera were absorbed with excess HCG, which abolished the reaction with HCG and HLH but still permitted assay of TSH. Bovine TSH had only slight cross-reaction with human TSH in the radioimmunoassay for HTSH, and therefore results could not be expressed in terms of the bovine USP reference standard. Results are expressed in terms of mμg of our laboratory human TSH standard, which contained 10 USP U/mg by bio-assay in the chick at the outset of these studies. Serum from 50 euthyroid, 10 hypophysectomized and 12 pregnant adults contained less than 3 mμg TSH/ml of serum (assuming 10 USP U/mg, this is equivalent to less than 3 mU/100 ml). Serum extracts from 11 additional euthyroid patients contained from 0.6 to 2.8 mμg/ml (0.6–2.8 mU/100 ml) with a mean of 1.8 μg/ml. Serum from 10 hyperthyroid patients also containedless than 3 mμg TSH/ml. The possible significance of this in terms of etiology of thyrotoxicosis is discussed. Sera from 13 patients with primary myxedema contained 7–156 mμg/ml (7–156 mU/100 ml). These values fell toward normal with appropriate replacement therapy. Serial measurements of TSH levels in a myxedematous patient undergoing replacement treatment with graded doses of l-thyroxine are presented. Treatment of 6 euthyroid patients with 30–45 mg of methimazole daily for 10–14 days led to elevations in TSH levels in 4 of these patients to 2–10 times normal and produced no changes in 2 patients. In the 2 patients who had no elevation in TSH levels, no changes in PBI were observed. The elevated TSH levels were suppressed to undetectable levels after intravenous treatment with l-thyroxine.