Calcitonin deficiency in primary hypothyroidism.

The relative lack of sensitivity and specificity of current assays for plasma calcitonin (CT) have made it difficult to study possible CT deficiency conditions. Using a new extraction method that considerably improves the sensitivity of the assay for CT monomer, CT levels were measured before and after a short calcium (Ca) stimulation test (2 mg Ca/kg over 5 min) to determine C-cell secretory reserve in women with autoimmune primary hypothyroidism and normal women. Mean basal plasma CT concentrations were lower in the hypothyroid women [0.9 +/- 0.1 (+/- SEM) pg/ml] than in the normal women (1.5 +/- 0.2 pg/ml; P less than 0.01). Serum Ca increased similarly in both groups, but postinfusion CT levels were lower in the women with primary hypothyroidism (3.8 +/- 1.3 pg/ml) than in normal women (15.9 +/- 3.0 pg/ml; P less than 0.001). The functional thyroid status at the time of the study did not influence CT levels; both hypothyroid patients (n = 10) and patients who were euthyroid during T4 treatment (n = 11) were CT deficient to the same extent. Unlike that in primary hypothyroidism, CT secretion was normal in four patients with hypothyroidism of pituitary origin. We conclude that the process that causes hypothyroidism in patients with autoimmune thyroid disease can also cause marked CT deficiency. This first demonstration of spontaneous CT deficiency in adults should contribute to the understanding of CT physiology; it also suggests that bone metabolism should be closely monitored during the treatment of primary hypothyroidism.