In this study, we performed PTH(1–84) measurement in a relatively large group of PHP1 patients and compared the conformity of PTH(1–84) assay with iPTH assay. Results showed that similar to that of in the normal control group, PTH(1–84) assay correlated well with iPTH in both PHP1 and PHPT groups. Conformity test showed a good performance of the two assays in the determination of elevated PTH level in PHP1 patients, while a relatively poor conformity of these two assays in PHPT patients. Moreover, the two assays showed similar power in evaluating elevated PTH level in different subtypes of PHP1 patients.
Ever since the establishment of PTH(1–84) assay, its reliability and potential usefulness in estimating parathyroid function have been tested and confirmed by various groups [8, 13, 14]. The validity of PTH(1–84) assay in our study was supported by the good correlation with the iPTH assay and the similar ability in determining higher than normal level of PTH when comparing with iPTH assay. However, the differences between the two assays shouldn’t be neglected. According to the different epitopes of the antibodies used in the two assays, the PTH fragments of various lengths in the circulation were responsible for the difference between them. In our research, both the two indexes of ∆PTH% and 3rd /2nd ratio showed that approximately a quarter of iPTH concentration in PHP1 and PHPT patients was made up of PTH fragments, which was significantly higher than in normal controls (about 15%). Previous studies so far had also noticed that the 3rd /2nd ratio differed among diseases, though most of them focused on one disease group at one time. It has been reported that the ratio was 0.60–0.62 in CKD patients in different studies [15–17], while the ratio in patients with PHPT was reported to be 0.60–0.64 [18, 8]. Only one single-center observation conducted the two assays among different patient groups and showed that the medium 3rd /2nd ratio for hemodialysis patients, renal transplantation recipients, PHPT patients, parathyroid carcinoma patients, and healthy elder people (medium age was 72.6 years) was 0.74, 0.77, 0.76, 1.16, and 0.80 respectively [19]. Further examinations of the comparisons of these two assays are needed to conduct for a deeper knowledge of the 3rd /2nd ratio and the differences between assays.
Due to the rarity of PHP, the studies about the comparison of these two assays in such patients are very limited. Hatakeyama et al had conducted the two assays on seven newly diagnosed PHP1 patients and healthy volunteers, and they found that PHP1 patients presented with a significantly higher proportion of PTH(7–84)-like fragments to PTH(1–84) when comparing with the normal control group, and the 3rd /2nd ratio was lower in PHP1 group than in normal control (average 3rd /2nd ratio was 0.64 and 0.77, respectively, p < 0.01) [10]. In our study, we conducted the iPTH and PTH(1–84) measurement simultaneously in an apparently larger group of PHP1 patients with clear molecular subtyping, who had received treatment of calcium and calcitriol. We observed also lower 3rd /2nd ratio in PHP1 patients than in normal controls, but relatively higher ratios in both groups (means of 0.76 and 0.84, respectively) than previous study. The discrepancy of the results may be due to the differences such as sample size, assay kits, etc. Decay during the storing of samples as well as the treatment of calcium and calcitriol that all of our patients were receiving may also contribute to the difference between these two studies [10]. We further compared the 3rd /2nd ratio and conformity of the two assays in two subgroups of PHP1A and PHP1B classified by molecular analysis and found no significant differences. It was suggested that inactivating mutation (PHP1A) and methylation alteration (PHP1B) of GNAS had not led to the difference in the proportion of PTH fragments in the circulation. Studies with more fresh serum samples and patients with different treatment status are needed to fully evaluated the clinical application of PTH (1–84) in PHP1 patients.
Considering the difference in mechanisms of PTH elevation in PHP1(PTH resistance with reserved PTH responsiveness to serum calcium change) [20, 21] and PHPT (autonomous production and secretion of PTH by parathyroid adenoma), it had been speculated that there may be a difference in PTH degradation leading to changes in PTH fragments in circulation in these two clinical entities. We did observe a better performance in the conformity test of the two assays in the judgement of elevated PTH values and higher r2 in linear regression in PHP1 patients than in PHPT patients. However, the calculated ∆PTH% and 3rd /2nd ratio were of no statistical differences between the two groups, indicating that PHP1 and PHPT patients possessed a relatively similar proportion of PTH fragments in the circulation. Previous researches unveiled that PTH fragments of various lengths were mostly the product of liver, while part of them was secreted directly by parathyroid [1, 2], and their clearance relied on proper renal function. Recent research involving predialysis CKD patients found that the differences between iPTH and PTH(1–84) increased along with the degree of renal function impairment and amount to about 30% in these patients [22]. Though eGFR levels were different between groups, all the patients enrolled had a normal renal function, which would be a reason for the similar ∆PTH% and 3rd /2nd ratio between PHP1 and PHPT. Although with similar means of calculated ∆PTH% and 3rd /2nd ratio, a greater variation was seen in these two indexes in PHPT patients, which may cause the differences in the conformity test. The differences in severity or the pathology (such as parathyroid carcinoma) of the disease may be responsible for the relatively poor conformity in PHPT group, though further investigations are needed to support the hypothesis.
Interestingly, there were 9 PHPT patients with the PTH(1–84) values higher than iPTH values. Confirmation of medical history revealed that among these nine patients, six patients were diagnosed with parathyroid carcinoma (PC), one had atypical adenoma. Another one patient later presented with hypergastrinemia and pancreatic occupation, which indicated the diagnosis of multiple endocrine neoplasia type 1 (MEN1). One patient didn’t undergo surgery. Our finding agreed with a former study using an automated PTH evaluation platform which featured PC with an average 3rd /2nd ratio > 1 while the ratio was < 1 in their control group of with benign parathyroid adenoma or hyperplasia [19]. A higher 3rd /2nd ratio and a greater proportion of patients with a 3rd /2nd ratio > 1 have been demonstrated in several groups previously [23–25]. Another study presumed that the overproduced N-terminal molecular PTH in PC patients was responsible for the relatively higher PTH(1–84) value [18]. Though investigations showed that the N-terminal fragments were also concentrated in CKD patients [26], the normal eGFR level of the six PC patients in our study suggested that the hyperplastic parathyroid rather than the declination of renal function was responsible. The potential mechanism of the similar 3rd /2nd ratio that the MEN1 patient presented here was still unknown, which should be further observed in more cases of MEN1 and to explore the exact mechanism.
This study was the ever first investigation that compared the conformity of PTH(1–84) with iPTH among the PHP1patients. The two assays were conducted with clinically verified diagnostic kits on automated platform in succession, thus making the results highly reliable. Another strength was that the investigation was conducted on a relatively large population of Chinese PHP1 patients. However, the apparent declination of the post-storing iPTH when compared with pre-storing indicated that the interpretation of samples after long-term storage should be done carefully. The declination of PTH(1–84) between pre-storing and post-storing as well as the conformity test between PTH(1–84) assay and iPTH assay before storing could have been done if our center had been equipped with PTH(1–84) assay the time all samples were collected. Considering the rarity of the disease and the influence of the COVID-19 pandemic on patients’ visiting, the sample size of PHP1 patients is rather small. Another limitation of this study was that all the PHP1 patients were taking medical treatment of calcium and calcitriol. Further recruitment of newly diagnosed PHP1 patients will be of necessary for a clearer evaluation of basic PTH level among them.