Radioimmunoassay of urinary calcitonin in normal subjects and in patients with medullary carcinoma of the thyroid.

A simple technique for measuring urine calcitonin (CT) was established using a sensitive radioimmunoassay (RIA) system for plasma human calcitonin (hCT). To extract urinary CT, urine samples were fractioned by gel chromatography on a column (0.8 x 20 cm) of Bio Gel P-2. Recovery of synthetic (1-32)hCT was 86.1 +/- 6.2% and the intra- and inter-assay coefficients of variation in RIA were 5.9 and 8.2%, respectively. Dilution curves of the urinary CT after gel-filtration were parallel with the standard curve. In 11 patients with medullary carcinoma of the thyroid (MCT), the CT levels of the urine (in ng/mg Cr) were 3.4 to 20.8 times higher than those of the plasma (in ng/ml), and a significant positive correlation (r = 0.93, P less than 0.001) was obtained between the urinary and plasma levels of CT in these 11 patients. In 32 normal subjects, the CT levels of the urine (in pg/mg Cr) were only 0.41 to 5.1 times of those of the plasma (in pg/ml), and a weak positive correlation (r = 0.408, P less than 0.05) was obtained between these two levels. However, urinary CT level in normal subjects apparently reflected a rise in endogenous plasma hCT in response to calcium infusion (4 mg/kg B.W. for 1 min) and that of exogenous plasma CT when synthetic (1-32)hCT (500 micrograms for 1 min) was injected intravenously. Following gel filtration on a Bio Gel P-30 1.5 x 80 cm column, larger molecular forms of CT than (1-32)hCT were found in the urine not only patients with MCT but also of normal subjects after infusion of calcium or synthetic (1-32)hCT. It is concluded that measurement of urine CT is useful not only for diagnosis of MCT but also for evaluating CT metabolism in normal subjects.