Thyrotoxicosis, hypercalcemia, and secondary hyperparathyroidism.

Two patients with hypercalcemia and hyperthyroidism had elevated levels of parathyroid hormone (PTH). When the patients were made euthyroid with appropriate medical therapy, both the levels of PTH and calcium returned to normal. Since thyroid hormone can increase tissue responsiveness to catecholamines, and since catecholamines can stimulate PTH secretion, we postulate that the elevated levels of PTH were secondary to thyrotoxicosis. In patients with coexisting hyperthyroidism and hyperparathyroidism, primary hyperparathyroidism should only be diagnosed when the patient is eumetabolic.