Radioimmunoassay of calcitonin in unextracted human serum: a sensitive method

A sensitive sequential direct radioimmunoassay of calcitonin (CT) is described. Human synthetic monomer CT was used for 125I-labelling and for preparation of standard sera. The antibody (rabbit) was directed against mid-portion and C-terminal parts of the human CT molecule. No significant cross reactivity with salmon or porcine CT was found. Separation of free from antibody-bound 125I-labelled CT was performed using a double antibody-polyethyleneglycol mixture. Dilution curves of sera from normal controls and patients with medullary thyroid carcinoma (MCT) were parallel to the standard curve. Recoveries of 25-400 ng/l CT were in the range of 88-138%. The intraassay coefficients of variation (CV) were 6% and 3% at serum concentrations of 90 and 230 ng/l respectively. The interassay CV were 13% and 10% at mean serum CT concentrations of 50 and 210 ng/l respectively. The lower limit of detection was 20 ng/l. Thirty-one healthy controls had serum CT levels in the range of 35-135 ng/l. Elevated basal serum CT concentrations were found in all of 11 patients with MCT and the serum concentrations increased markedly after pentagastrin stimulation. Subsequent thyroidectomy in eight of the 11 patients with MCT reduced basal serum CT levels, in six to within the reference range for normals.