Elevated serum calcitonin levels in patients with thyroid disorders

Serum calcitonin was measured by radioimmunoassay (RIA) in 63 patients with hyperthyroidism, 37 with hypothyroidism, 9 with surgically proven medullary thyroid carcinoma (MTC) and 81 normal subjects. Provocative tests of calcitonin by calcium infusion were performed in 11 with hyperthyroidism, 10 with hypothyroidism, 6 with MTC and 14 normal subjects. Changes of calcitonin were also studied before and after treatment in 17 patients with hyperthyroidism and 12 with hypothyroidism. The basal calcitonin levels were increased in both hyperthyroidism (117 +/- 60.1 pg/ml (mean +/- SD): n = 63, P less than 0.001) and hypothyroidism (137 +/- 107 pg/ml, n = 37, P less than 0.001), and significantly increased in MTC 14 765 +/- 25 039 pg/ml, range 390 to 70 400 pg/ml, n = 9, P less than 0.001) compared with those in normal subjects (45.8 +/- 22.3 pg/ml, n = 81). Increases of calcitonin during calcium load (iv 4.5 mg calcium/kg for 10 min) were significantly lower in both hyperthyroidism and hypothyroidism patients than in those in MTC. The calcitonin levels were not correlated with the serum T3, T4, TSH concentrations or titres of serum antithyroid antibodies in hyperthyroidism and hypothyroidism. We found some tendency toward decreasing calcitonin levels with the euthyroid state after treatment in both groups, but the changes were not significant. Although exact mechanisms of increased calcitonin in hyperthyroidism and hypothyroidism were not clear, our findings suggest that a 2- to 3-fold elevation of calcitonin levels in these patients did not indicate malignancy; MTC could be readily differentiated by a calcium infusion test.