Concurrent hyperthyroidism and hyperparathyroidism: influence of hyperthyroidism on serum magnesium, free calcium and parathyroid hormone.

A patient with concomitant primary hyperparathyroidism and Graves' disease with hyperthyroidism was observed during treatment with carbimazole. His serum free calcium levels was initially elevated (6.3mg/dl, normal 4.5-5.3 mg/dl) whereas his serum magnesium level was depressed (1.6 mg/dl, normal 1.7-2.8 mg/dl), but immunoreactive parathyroid hormone levels were only slightly increased (96 pg/ml, normal less than 86 pg/ml). During therapy with carbimazole, the patient became euthyroid and serum free calcium levels decreased to 6.0 mg/dl whereas serum magnesium levels increased to 2.3 mg/dl, and parathyroid hormone increased to values clearly indicative of hyperparathyroidism (185 pg/ml). A subtotal thyroidectomy and parathyroidectomy corrected both diseases in this case. These observations suggested that hyperthyroidism per se causes a decrease in serum magnesium levels and also potentiates the osteoclastic effects of parathyroid hormone resulting in an exacerbation of hypercalcemia which in turn produced a relative inhibition of hormone secretion by the abnormal parathyroid tissue.