Heterogeneity of immunoreactive calcitonin in extracts of normal human thyroid

Immunoreactive calcitonin in serum of patients with medullary carcinoma of the thyroid and in extracts of this tumor has been found to exhibit heterogeneity when analysed by gel-chromatography and radioimmunoassay. In this study normal human thyroid tissue was homogenized in buffered saline a neutral ph and a soluble fraction was obtained after ultracentrifugation. Extracts were analysed by gel-filtration before and after treatment with detergents, with denaturing agents and after reduction and alkylation. The stability in vitro of the extracted immunoreactive calcitonin was determined at different temperatures and the changes in heterogeneity analysed by gel-filtration. The results suggest that at least part of the largest form of immunoreactive calcitonin is an artefact of the chromatographic procedure. Moieties with a molecular weight below 12000 Dalton were present and could be identical with prehormones described by others. However, no indication for a sequential degradation releasing calcitonin monomer from larger precursors was found in normal thyroid.