Parathyroid hormone (PTH) is a single-chain polypeptide made up of 84 amino acids. The ability to measure PTH levels has only been available since the 1960s, when an immunoassay was developed by Berson and Yalow, a discovery that led to the awarding of a Nobel Prize to these investigator (
4). When PTH is metabolized by proteolysis in the liver, kidneys, bone and, to a lesser extent in the parathyroid glands, it results in N-terminal, C-terminal and midregion fragments. The N-terminal region contains the portion that is bioactive, and disappears rapidly. The C-terminal fragment has a half-life of several hours. Immunoassays specific for various PTH fragments rely on antiserum specific for a portion of the polypeptide. More recently, assays for iPTH are widely commercially available and are now being used to measure Io-PTH ( 5).
Older assays for rapid-PTH used an antibody developed to bind to the nPTH fragment of the hormone. While these assays were initially thought to bind to the first 34 amino acids or (1-34) PTH, it has since been discovered that the anti-nPTH antibody used in these assays binds only to amino acids 13-34, or (13-34) PTH (
6). This limiting factor in detecting iPTH, or (1-84) PTH, might explain why Io-PTH levels reported in earlier studies saw only modest (50%) decrease in Io-PTH levels, even after successful surgical resection of all diseased glands. A 50% drop of Io-PTH from baseline within 5-10 minutes after excision of an abnormal gland had been reported in earlier studies to be predictive of a cure in 95% of patients. This threshold of a 50% decline in Io-PTH levels may no longer then be appropriate if second and third-generation tests that measure iPTH or bio-intact (1-84) PTH are used. Older assays may not be measuring biologically active hormone alone, instead, measuring longer lasting fragments of the metabolized hormone, and therefore may not decline as much or be as biologically accurate.
In addition to assays for the metabolized fragments of PTH, assays for iPTH attempt to more completely identify the intact 84 amino acid hormone. In this study, we used a commercially available, automated, multipurpose analyzer (Immulite 1000 with Turbo intact PTH kit) with highly specific antibodies for iPTH. The assay requires binding by both a solid-phase antibody specific for the C-terminal region [(44-84) PTH] and by an enzyme-labeled antibody that only recognizes the N-terminal region [(1-34)PTH], and therefore is able to only recognize iPTH and very large PTH fragments that are nearly as long as the iPTH molecule itself. One such large fragment, 7-84 PTH, can cross react, but the clinical significance of this potential cross reaction is unknown. In 2002, Kao et al reported that 45 of 47 patients undergoing parathyroidectomy for PHPT using this same assay had iPTH levels decrease to < 25% of baseline levels (
We report a large, consecutive, experience measuring Io-PTH during procedures for PHPT. The results confirm the hypothesis that Io-PTH levels decline to normal, or near-normal, within 10-20 minutes, when all abnormal parathyroid glands are removed at surgery. We observed that Io-PTH levels decline to the normal reference range in 90% of patients, or by more than 80% of the initial level in 96% of patients at the conclusion of surgery for PHPT. Levels that declined by 50%, but were still significantly elevated, would be interpreted by the authors to indicate the need for further, bilateral, four-gland, exploration of the neck in an attempt to find additional enlarged, hyperfunctioning glands. Given the known short half-life of bioactive (1-84) PTH of only 2-5 minutes, persistently elevated levels of iPTH 10-20 minutes after resection of an abnormal gland strongly suggest additional diseased gland(s).
The ability to measure intraoperative parathyroid hormone (Io-PTH) has supported the increasing use of unilateral exploration in selected patients with primary hyperparathyroidism (PHPT), and is felt to be especially important for these directed or limited surgical approaches. Combined with preoperative localization studies, especially 99Tc-labeled sestamibi imaging, 70% of patients in the current study were able to undergo only unilateral neck exploration, yet serum calcium returned to normal in 96% of study patients. Conversely, a persistent elevation of Io-PTH levels after resection of an abnormal gland led to otherwise unplanned bilateral exploration in 22 patients (11%), emphasizing the value of measuring Io-PTH if less than a full, bilateral, four-gland exploration is planned (
Given the short half-life of bio-intact (1-84) PTH, Io-PTH levels of iPTH should become normal, or to drop by at least 80%, during successful parathyroidectomy for PHPT. Normal or near-normal post-resection Io-PTH levels can support unilateral exploration in PHPT. Persistent elevations of Io-PTH after resection of an abnormal gland dictate the need for bilateral exploration. As assays become more specific for iPTH, and thus more accurately reflect biologically active PTH hormone in the blood, the previous standard of a 50% decline of Io-PTH levels is no longer appropriate.