Concomitant graves' disease and primary hyperparathyroidism. Influence of hyperthyroidism on serum calcium and parathyroid hormone

Two patients with coexistent Graves' disease and primary hyperparathyroidism were studied during medical treatment of their hyperthyroidism. Serum free calcium level was initially quite elevated (1.61 and 1.71 mM, normal 1.12 to 1.28 mM), but immunoreactive parathyroid hormone values were only slightly increased. The immunoreactive parathyroid hormone values of 153 and 173 nleq/ml (normal less than 150 nleq/ml) were far lower than expected in hyperparathyroid patients with a similar degree of hypercalcemia. As the patients became euthyroid during thionamide treatment, calcium values decreased to 1.39 and 1.61 mM, respectively, and parathyroid hormone increased to values clearly suggestive of hyperparathyroidism (454 and 229 nleq/ml, respectively). Parathyroidectomy and subtotal thyroidectomy cured both the hyperparathyroidism and the thyrotoxicosis in each case. These observations suggest that thyroid hormone had potentiated the osteoclastic effects of parathyroid hormone and that the resulting exacerbation of hypercalcemia had produced a relative suppression of hormone secretion by the abnormal parathyroid tissue.