Effect of cimetidine on circulating parathyroid hormone in primary hyperparathyroidism

In twelve patients with primary hyperparathyroidism the raised parathyroid hormone (PTH) levels consistently returned to normal with cimetidine ("Tagamet"), 300 mg four times a day, orally. As PTH levels returned to normal, signs and symptoms improved. Reduction of hypercalcaemia varied with the time, dose, and route of administration of cimetidine. Whenever cimetidine was discontinued, there was a rebound of PTH levels to approximately twice the initial value, which suggests that cimetidine blocks the synthesis and/or secretion of parathyroid hormone, so that parathyroid hormone or its precursor accumulates in the gland. A solitary parathyroid adenoma has been confirmed surgically in the five patients so far operated on. In all surgical patients the PTH levels were brought to normal by cimetidine preoperatively; these levels were maintained postoperatively. The adenomas were unusually large, firm, and congested; there was no atrophy of the rest of the glands. These last two observations further support our hypothesis that cimetidine blocks the synthesis or release of parathyroid hormone; they also support the concept that in primary hyperparathyroidism the parathyroid adenoma becomes autonomous or there is an abnormal hormone.