Radioimmunoassay for human calcitonin employing synthetic calcitonin M: its clinical application.

A sensitive and reliable radioimmunoassay for human calcitonin was described and applied to preliminary clinical studies. 125I-labelled synthetic human calcitonin M was purified by gel filtration with Sephadex G-25 and G-100. A nonequilibrium incubation system was applied at the final volume of incubation mixture of 500 mul, in which pooled plasma from normal subjects or hormone free serum was used as carrier protein at 20% incubation mixture. Dextran T 40 coated charcoal, resuspended in 1% bovine serum albumin buffer, was used for the separation of free from bound tracer. This showed the least nonspecific adsorption of tracer to charcoal. The assay was sensitive to 0.1 ng per milliliter of plasma. Recovery of synthetic human calcitonin added to plasma was found to be 101% (S. D., +/- 8). Diluted plasma from a patient with medullary thyroid carcinoma showed a dilution curve parallel to standards. Basal calcitonin levels were less than 0.3 ng/ml in normal subjects. Relatively high values were found in patients with chronic renal failure and in patients with malignant tumors. Extremely elevated values were found in patients with medullary thyroid carcinoma. Provocative calcium infusion tests were also performed.