Immunoheterogeneity of parathyroid hormone pre- and postoperatively in primary hyperparathyroidism

In primary hyperparathyroidism (pHPT), a preferential release of intact PTH (i-PTH) versus carboxylterminal PTH fragments is known to occur. We studied whether the release of amino-terminal PTH fragments (N-PTH) is also changed. Serum levels of i-PTH and N-PTH were determined under basal conditions and following oral intake of calcium in six patients with pHPT before and immediately after surgery and in seven healthy subjects. In the patients, baseline levels of both i-PTH and N-PTH were increased preoperatively. The increase was larger in i-PTH compared to N-PTH. Therefore, the N/i ratio was reduced compared to healthy subjects (P < 0.05). On the first postoperative day, serum i-PTH decreased to a larger extent than N-PTH, which increased the N/i ratio above that in healthy subjects (P < 0.05). On the 5th postoperative day, the N/i ratio was normalized. Preoperatively, the suppressibility of i-PTH calcium was impaired in the patients (P < 0.05), whereas the suppressibility of N-PTH was normal, resulting in unchanged N/i ratio during the oral calcium load. In contrast, the N/i ratio increased normally during the calcium load at day 5 postoperatively (P < 0.05). We therefore conclude that: (1) in pHPT, circulating PTH immunoheterogeneity is altered with a preferential release of intact PTH compared to N-terminal PTH fragments and this alteration is normalized after surgery, (2) the secretion of intact PTH and N-terminal PTH shows different sensitivity to inhibition by calcium.