Immunoreactive forms of circulating parathyroid hormone in primary and ectopic hyperparathyroidism

Area of Science:

• Endocrinology
• Biochemistry
• Oncology

Background:

• Hyperparathyroidism involves elevated parathyroid hormone (PTH) levels.
• Distinguishing between primary and ectopic hyperparathyroidism is crucial for diagnosis and treatment.
• Previous studies noted lower total immunoreactive PTH (iPTH) in ectopic hyperparathyroidism.

Purpose of the Study:

• To compare the molecular forms of immunoreactive parathyroid hormone (iPTH) in primary and ectopic hyperparathyroidism.
• To investigate the reasons for differences in total iPTH levels between these two conditions.

Main Methods:

• Plasma samples from six patients with primary and six with ectopic hyperparathyroidism were analyzed.
• Gel filtration chromatography (Bio-Gel P-150) separated iPTH components by molecular weight.
• Radioimmunoassay quantified iPTH in eluted fractions.

Main Results:

• Ectopic hyperparathyroidism plasma had significantly lower levels of small COOH-terminal iPTH fragments (mol wt < 9,500) compared to primary hyperparathyroidism.
• The quantity of intact or large PTH forms (eluting with or before bovine PTH [1-84]) was similar in both groups.
• A large PTH component, not previously recognized, was found in both syndromes, potentially predominant in ectopic cases.

Conclusions:

• Lower circulating levels of biologically inactive COOH-terminal fragments contribute to lower total iPTH in ectopic hyperparathyroidism.
• Sufficient biologically active iPTH is present in both conditions to explain hypercalcemia.
• A significant, previously unrecognized large PTH component exists in both primary and ectopic hyperparathyroidism.