Abstract
Parathyroid hormone has been measured by radioimmunoassay in eight patients with hyperparathyroidism due to parathyroid adenoma. Secretion of hormone by the adenomatas was demonstrated direct by estimating the arteriovenous gradient of parathyroid hormone across the tumours. Serial estimations following surgical removal of the adenoma showed a rapid fall in the concentration of circulating parathyroid hormone. The calculated half-life of endogenously secreted parathyroid hormone in man varied from 11.4 to 28.8 min., with a mean of 19.8 min.