Studies of circulating parathyroid hormone in man by a homologous amino-terminal specific radioimmunoassay.

A homologous radioimmunoassay specific for amino-terminal portion of human parathyroid hormone(PTH) was developed in order to study the clearance of biologically active species of the hormone in the circulation in man. Characterization of the assay system with synthetic analogues of human PTH (1-34) indicated that the carboxyl-terminal region of human PTH(1-34) is an important recognition site. Plasma amino-terminal PTH levels were less than 0.3 ng/ml in all of 35 normal subjects. The levels were elevated above 0.3 ng/ml in 15 of 24 patients with primary hyperparathyroidism. In 5 patients in whom the levels were determined before and after parathyroidectomy, the elevated levels were all normalized within 60 minutes after the removal of adenomata. The disappearance of exogenous human PTH(1-34) was studied after intravenous administration in 17 patients with hypoparathyroidism. A graphical analysis of the data disclosed two major components of the disappearance curve with estimated half-disappearance time of 3 and 28 minutes respectively, suggesting that multiple mechanisms are involved in the clearance of the peptide from the circulation. These results demonstrate usefulness of homologous radioimmunoassay for human PTH(1-34) in diagnosis and management of hyperparathyroidism, as well as in studying the clearance of amino-terminal portion of PTH which is known to represent biological activity.