Carboxyl-terminal parathyroid hormone fragments: role in parathyroid hormone physiopathology

Stability and validity of intact parathyroid hormone levels in different sample types and storage conditions

BACKGROUND

Several pre-analytical factors can affect the measurement of intact Parathyroid Hormone (IPTH). In this study, we have investigated the effects of using different types of tubes, time elapsed before separation, and storage conditions over time on the measured values of IPTH.

METHOD

Blood samples from 30 subjects were collected into plain, SST, and EDTA tubes. All serum and plasma were separated immediately (first set) and after 2 hrs delay (second set). The first set of samples were aliquoted and stored at RT (25°C), at fridge (4°C), and freezer (-20°C). IPTH was measured in all the stored aliquots at 2,4, and 8 days after collection using Architect analyzer.

RESULTS

Paired T test and ANOVA repeated measures showed no significant difference between IPTH levels in all tubes. The second set of serum and plasma were significantly lower (3.8% and 7.4%, p < 0.001, respectively) when compared to samples measured initially. Serum samples stored at RT were significantly lower (by 45%,59%, and 77%) on days 2,4, and 8 when compared to the initial time (p < 0.001 in all cases). Plasma samples stored at RT, were significantly lower on day 8 after collection, by 30.8% (p < 0.001). These differences would be clinically important.

CONCLUSION

Plasma IPTH can be stored at RT for up to four days. Both plasma and serum IPTH are not affected by a delay in the separation of up to two h and they can be stored for up to 8 days in a fridge or freezer without any clinically significant changes in their values.

PTH determination in hemodialyzed patients-A laboratory perspective

Parathyroid hormone (PTH) is a key player of bone remodelling in patients suffering from Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD). Serum PTH concentrations are thus frequently measured in CKD patients. Nevertheless, this determination is far from simple. PTH stability can be an issue and degradation of the peptide can be important if storage is not properly done. Biologically active PTH circulates together with fragments, which can be detected by some immunoassays. There is, up to now, no standardization of the assays available on the market, which can lead to some confusion when patients are followed with different methods. The upper end of the reference ranges provided by some manufacturers have not been properly established and are sometimes far too high. Finally, PTH can be oxidized in vivo and thus become inactive, while still quantified by immunoassays. In this Editorial, we will try to highlight some of these issues on PTH measurement.

Parathyroid Hormone Measurement in Chronic Kidney Disease: From Basics to Clinical Implications

Accurate measurement of parathyroid hormone (PTH) is crucial for therapeutic decision-making in patients with chronic kidney disease-mineral and bone disorder (CKD-MBD). The second-generation PTH assays, often referred to as “intact PTH” assays, are the current standard and most available assays in clinical practice. However, intact PTH assays measure both full-length biologically active PTH and heterogeneous PTH fragments in the circulation, providing the equivocal value of PTH measurement in patients with CKD-MBD. Due to the variability of PTH assays, preanalytical sample errors, and the phenomenon of end-organ PTH hyporesponsiveness, current CKD-MBD guidelines recommend a wide range for serum PTH targets (2–9 the upper normal limit of the intact PTH assay) in dialysis patients to diminish the risk of developing adynamic bone disease. Nevertheless, a sizeable proportion of CKD patients still experience renal osteodystrophy despite having serum PTH levels within the recommended range. The primary cause of this inconsistency is the analytical interference of various PTH fragments and oxidized PTH forms that considerably accumulate in CKD patients. Therefore, a new mass spectrometry-based assay, which is capable of specifically measuring the whole spectra of PTH fragments, can potentially improve diagnostic accuracy for renal osteodystrophy. However, the effects of different PTH fragments on bone metabolism, vascular calcification, and mortality in CKD patients warrant further research.

Determining biological variation of serum parathyroid hormone in healthy adults

Introduction

Measurement of parathyroid hormone (PTH) is essential in the investigation and management of calcium metabolism disorders. To assess the significance of any assay result when clinical decision making biological variation (BV) of the measurand must be taken into consideration. The aim of the present study is determining the BV parameters for serum PTH.

Materials and methods

Blood samples were taken at weekly intervals from 20 healthy subjects for ten weeks in this prospective BV study. Serum “intact PTH” concentrations were measured with electrochemiluminescence method. Biological variation parameters were estimated using the approach proposed by Fraser.

Results

The values of within-subject biological variation (CV_I), between-subject biological variation (CV_G), analytical variation (CV_A), reference change value (RCV) and individuality index (II) for serum PTH were 21.1%, 24.9%, 3.8%, 59.4% and 0.8%, respectively. Within-subject biological variation and CV_G were also determined according to gender separately; 18.5% and 24.0%; 26.2% and 18.6% for male and female, respectively. Calculated desirable precision and bias goals were < 10.6% and < 6.3%, respectively.

Conclusion

This study may contribute to BV data on serum PTH as it includes a sufficient number of volunteers from both genders over an acceptable period of time. We do not recommend the usage of population-based reference intervals for serum PTH concentrations. Reference change value may be helpful for the evaluation of serial serum PTH results. Nonetheless, evaluation of data according to gender is necessary when setting analytical performance specifications.

PTH assays in dialysis patients: Practical considerations

Parathyroid hormone (PTH) 1-84 is the main biologically active hormone produced by the parathyroid cells. Circulating PTH molecules include the whole PTH 1-84 along with amino (N) and carboxyl (C) terminal fragments. While PTH is the best available noninvasive biomarker to assess bone turnover in dialysis patients, the biological roles of individual circulating PTH fragments are still not completely known. The understanding that there is an enormous variation in the target specificity of currently available PTH assays for different circulating forms of PTH has led to the evolution of assays from first to second then third generation. With a reduction in kidney function, there is a preferential increase in circulating C fragments and non-PTH 1-84 forms, resulting in a decrease in the ratio of PTH 1-84/non-PTH 1-84. However, there are also substantial differences in between-assay measurements, with several fold variations in results. Targets based on multiples of the upper limit of normal (ULN) should be used rather than PTH ranges using absolute iPTH values. To date, the second-generation PTH remains the most widely used assay. Current guidelines recommend following iPTH trends rather than absolute values. Herein, we highlight problems and challenges in PTH assays/measurements and their interpretations in dialysis patients.

Critical Governance Issue of Parathyroid Hormone Assays and its Selection in the Management of Chronic Kidney Disease Mineral and Bone Disorders

Measurement of circulating parathyroid hormone (PTH) levels is essential for optimal management of mineral and bone disorders (MBD) in chronic kidney disease (CKD) patients. There are two major types of PTH assays currently in use: intact parathyroid hormone (i-PTH) and whole PTH (w-PTH) assays. The i-PTH assay is the current standard, and considerable information regarding the management of CKD-MBD has been obtained with this method. However, several limitations have been found with the i-PTH assay. One limitation is that i-PTH assay also measures fragments other than full-length PTH (1-84). Another limitation is the existence of multiple readout methods of the i-PTH assay. The w-PTH assay is theoretically ideal because it exclusively detects full-length PTH (1-84). However, clinical data proving the advantages of w-PTH measurement are not sufficient. For uremic patients, Kidney Disease Improving Global Outcomes suggest that PTH levels should be maintained within approximately two to nine times the upper normal limit of the i-PTH assays. The most critical issue in the evaluation of PTH levels is the lack of definitive PTH assay method. Evidence-based recommendations on clinical management goals of PTH are warranted.

Comparison of Intact PTH and Bio-Intact PTH Assays Among Non-Dialysis Dependent Chronic Kidney Disease Patients

BACKGROUND

The third-generation bio-intact parathyroid hormone (PTH) (1-84) assay was designed to overcome problems associated with the detection of C-terminal fragments by the second-generation intact PTH assay. The two assays have been compared primarily among dialysis populations. The present study evaluated the correlations and differences between these two PTH assays among patients with chronic kidney disease (CKD) stages 3 to 5 not yet on dialysis.

METHODS

Blood samples were collected from 98 patients with CKD stages 3 to 5. PTH concentrations were measured simultaneously by using the second-generation - PTH intact-STAT and third-generation bio-intact 1-84 PTH assays. Other serum biomarkers of bone mineral disorders were also assessed. CKD stage was calculated by using the CKD-Epidemiology Collaboration (EPI) formula.

RESULTS

Serum bio-intact PTH concentrations were strongly correlated but significantly lower than the intact PTH concentrations (r=0.963, P<0.0001). This finding was consistent among CKD stages 3 to 5. PTH concentrations by both assays (intact and bio-intact PTH) positively correlated with urea (r=0.523, r=0.504; P=0.002, respectively), phosphorus (r=0.532, r=0.521; P<0.0001, respectively) and negatively correlated with blood calcium (r=-0.435, r=-0.476; P<0.0001, respectively), 25(OH) vitamin D, (r=-0.319, r=-0.353; respectively, P<0.0001) and the estimated glomerular filtration rate (r=-0.717, r=-0.688; P<0.0001, respectively).

CONCLUSIONS

Among patients with CKD stages 3 to 5 not on dialysis, the bio-intact PTH assay detected significantly lower PTH concentrations compared with intact PTH assay. Additional studies that correlate the diagnosis and management of CKD mineral and bone disorders with bone histomorphometric findings are needed to determine whether bio-intact PTH assay results are better surrogate markers in these early stages of CKD.

Biochemical Testing Relevant to Bone

Laboratory biochemical testing is critical to the clinical understanding of bone disorders. Patients with skeletal diseases have underlying themes in their pathophysiology that would be impossible to detect without biochemical assessment of serum and urine minerals, vitamin D, parathyroid hormone, parathyroid hormone-related peptide, and bone turnover markers. Bone disorders are caused by abnormalities in signaling pathways that affect bone formation and resorption. Therapies for common bone diseases were developed in direct response to underlying biochemical abnormalities.

Biotransformation and in vivo stability of protein biotherapeutics: impact on candidate selection and pharmacokinetic profiling

Historically, since the metabolism of administered peptide/protein drugs ("biotherapeutics") has been expected to undergo predictable pathways similar to endogenous proteins, comprehensive biotherapeutic metabolism studies have not been widely reported in the literature. However, since biotherapeutics have rapidly evolved into an impressive array of eclectic modalities, there has been a shift toward understanding the impact of metabolism on biotherapeutic development. For biotherapeutics containing non-native chemical linkers and other moieties besides natural amino acids, metabolism studies are critical as these moieties may impart undesired toxicology. For biotherapeutics that are composed solely of natural amino acids, where end-stage peptide and amino acid catabolites do not generally pose toxicity concerns, the understanding of biotherapeutic biotransformation, defined as in vivo modifications such as peripherally generated intermediate circulating catabolites prior to end-stage degradation or elimination, may impact in vivo stability and potency/clearance. As of yet, there are no harmonized methodologies for understanding biotherapeutic biotransformation and its impact on drug development, nor is there clear guidance from regulatory agencies on how and when these studies should be conducted. This review provides an update on biotherapeutic biotransformation studies and an overview of lessons learned, tools that have been developed, and suggestions of approaches to address issues. Biotherapeutic biotransformation studies, especially for certain modalities, should be implemented at an early stage of development to 1) understand the impact on potency/clearance, 2) select the most stable candidates or direct protein re-engineering efforts, and 3) select the best bioanalytical technique(s) for proper drug quantification and subsequent pharmacokinetic profiling and exposure/response assessment.

Parathyroid hormone measurement in chronic kidney disease--an evolving issue for the nephrologist and the clinical laboratorist: minireview

Parathyroid hormone (PTH) is the polypeptide hormone produced by the parathyroid glands, which plays a central role in calcium homeostasis. Circulating PTH must be measured regularly in patients with chronic kidney disease (CKD)--mineral and bone disorders (MBD) to monitor and to adapt treatment with the aim of maintaining PTH levels within a defined narrow range of optimal values for each stage of CKD. Often, for the nephrologists, it is not easy to determine what PTH levels are clinically appropriate. Moreover, the PTH determination also shows many criticisms from the laboratory point of view and there is a clear need to standardize PTH measurements in every phase of the process: pre-analytical, analytical and post-analytical. In this review, all these aspects are summarized with particular reference to the most recent opportunities to improve PTH assays quality on the whole. To this aim, a closer cooperation between nephrologists and clinical laboratories is undoubtedly necessary.

Optimizing bone health in chronic kidney disease

Phosphocalcic metabolism disorders often complicate chronic kidney disease (CKD) and worsen as kidney function declines, with a consequence on bone structural integrity. The risk of fracture exceeds that of the normal population in both patients with pre-dialysis CKD and end-stage renal disease (ESRD). The increasing incidence of CKD, the high mortality rate induced by hip fracture, the decreased quality of life and economic burden of fragility fracture make the renal bone disorders a major problem of public health around the world. Optimizing bone health in CKD patients should be a priority. Bone biopsy is invasive. Dual-energy X-ray absorptiometry, commonly used to screen individuals at risk of fragility fracture in the general population, is not adequate to assess advanced CKD because it does not discriminate fracture status in this population. New non-invasive three-dimensional high-resolution imaging techniques, distinguishing trabecular and cortical bone, appear to be promising in the assessment of bone strength and might improve bone fracture prediction in this population. Therapeutic intervention in the chronic kidney disease-mineral and bone disorders (CKD-MBD) should begin early in the course of CKD to maintain serum concentration of biological parameters involved in mineral metabolism in the normal recommended ranges, prevent the development of parathyroid hyperplasia, prevent extra-skeletal calcifications and preserve skeletal health. In this paper, we review studies of mineral and bone disorders in patients with CKD and ESRD, the utility of current techniques to assess bone health and the preventive and therapeutic strategies for managing CKD-MBD.

Estimation of the stability of parathyroid hormone when stored at -80 degrees C for a long period

BACKGROUND AND OBJECTIVES

Stability of parathyroid hormone (PTH) at -80 degrees C for long storage periods has never been studied. This can be of importance for the conclusions of studies where blood banks have been constituted. The study's aim was to evaluate stability of PTH when stored as serum or plasma EDTA samples at -80 degrees C.

DESIGN, SETTING, PARTICIPANTS & MEASUREMENTS

Samples were collected from 16 chronic hemodialysis patients using EDTA and gel-separator tubes. Plasma and serum were aliquoted; one aliquot was assayed with Elecsys and Liaison methods to determine the "baseline" values and another aliquot after 1, 3, 6, and 12 mo. The factors "method," "tubes," "subjects," and "time" were included in a mixed linear model to evaluate their effects on measured PTH values. The prediction interval methodology was used to assess where a future result could be obtained with a defined probability.

RESULTS

With the Liaison method, the maximum storage times with either dry or EDTA tubes were estimated to be 9 and 2 mo, respectively. With the Elecsys method, samples could be stored at least 2 yr with acceptable level of degradation.

CONCLUSION

PTH stability at -80 degrees C is not infinite. Maximum storage time and acceptance limits (30%) were defined, showing that with one method, samples should be stored for not more than 2 mo, whereas the other could be stored for up to 2 yr. With any PTH assay, the maximum storage time should be evaluated to ascertain that samples will keep their initial reactive profile after prolonged storage periods.

A bicistronic expression strategy for large scale expression and purification of full-length recombinant human parathyroid hormone for osteoporosis therapy

Parathyroid hormone (PTH) contributes to the increase of trabecular connectivity and is a candidate medication for effective treating osteoporosis. PTH is a protein of 84 amino acids and some studies have suggested that the active site lies within the range from amino acid (aa) 1 to 34. However, a few reports have indicated a causal relationship between PTH (aa 1-34) and osteogenic sarcoma in rats, while some less obvious but important roles of the carboxyl-terminus of PTH were also found. Unfortunately, it is difficult to obtain the active integrated PTH (1-84) in vitro, due to the instability of both the protein and its mRNA. Because an alternative translation start site is located at +25 nucleotides downstream of the true start site, a truncated PTH can be translated. We constructed a rhPTH bicistronic expression plasmid (pTrepth) that could highly express non-fusion soluble rhPTH proteins in Escherichia coli. The BL-21(DE3) containing pTrepth was cultured on a small scale until satisfactory expression and purification results were obtained. We then amplified the transformed cells in a 15-L fermentor and harvested 27g/L cells (wet weight). Extensive rhPTH purification was achieved by a three step chromatography process. Activity tests demonstrated that our purified protein could dramatically increase cAMP in osteosarcoma cells in vitro.

Critical issues of PTH assays in CKD

Measurement of bioactive parathyroid hormone (PTH) is essential for the optimal management of secondary hyperparathyroidism and its associated bone disorders in chronic kidney disease (CKD) patients. For this purpose, three generations of increasingly specific PTH assays have been developed over the last 4 decades. To date, however, only second-generation PTH assays are most widely used, although these have been shown to cross-react with large PTH fragments having a partially preserved N-structure, mostly PTH(7-84). The newly developed third-generation PTH assays are believed to be the most specific means of measuring PTH(1-84), but their clinical utility remains debatable. More recently, these latter assays have also been shown to react with a new N-form of PTH, which has been identified in patients with severe hyperparathyroidism and parathyroid carcinoma. Progressive research in this area has advanced our understanding considerably regarding the circulating molecular forms of PTH and their pathophysiological roles in bone abnormalities associated with CKD. However, developing an ideal PTH assay continues to be difficult because of key issues such as the reliability of PTH as a surrogate marker for bone turnover, practicality of employing third-generation PTH assays, and unknown biological implications of N-PTH and other PTH fragments. Further research exploring these issues is mandatory to understand and optimally manage parathyroid disorders and bone abnormalities in CKD patients.

Update on parathyroid hormone: new tests and new challenges for external quality assessment

It is now 43 years since Berson and Yalow published the first radio-immunoassay (RIA) for parathyroid hormone (PTH) [S.A. Berson, R.S. Yalow, G.D. Aurbach, J.T. Potts, Immunoassay of bovine and human parathyroid hormone. Proc Natl Acad Sci U S A 49 (1963) 613-617] [1]. Since then, there have been marked advances in our understanding of this peptide hormone, its mechanism of action and biological regulation [J.T. Potts, Parathyroid hormone: past and present. J. Endocrinol. 187 (2005) 311-325] [2]. PTH has become a routine assay in tertiary care hospitals and is an essential element in the management of chronic kidney disease, parathyroid disorders and the investigation of abnormalities in calcium homeostasis. Despite continuing technological advances in PTH measurement, analyte heterogeneity remains a problem, while improved turnaround time and better precision are constantly escalating clinical demands. This mini-review begins with a brief update of current knowledge on PTH, followed by a summary of a recent Ontario-wide External Quality Assurance (EQA) survey, and concludes with comments on utilization trends, current and future.

Effect of haemodialysis on markers of bone turnover in children

'Intact' parathyroid hormone (iPTH) assays are used to measure serum PTH levels in haemodialysis patients to diagnose and monitor secondary hyperparathyroidism and consequent renal osteodystrophy (ROD); these assays exhibit cross-reactivity with long carboxyl-terminal PTH fragments (C-PTH) that accumulate in end stage renal failure (ESRF) and antagonise the biological activity of the whole molecule, 1-84 PTH. The effects of haemodialysis on C-PTH are not known. We investigated how haemodialysis affects serum concentrations of calcium, iPTH, 1-84 PTH, C-PTH, and other markers of bone turnover; bone-specific alkaline phosphatase (BALP) and type 1 collagen cross-linked telopeptide (CTx). Fifteen patients, mean (range) age 13.9 (4.3-17.6) years, haemodialysed for a median of 16.3 (4-41) months, had pre- and post-dialysis serum samples collected for routine biochemistry, BALP, CTx, iPTH and 1-84 PTH assays. Changes to serum concentrations and relationships between these biochemical surrogate markers of ROD were investigated. Serum phosphate and PTH levels (measured by both assays) fell significantly during dialysis, whereas serum calcium, C-PTH, the 1-84 PTH: C-PTH ratio and BALP and CTx concentrations were not significantly changed. 1-84 PTH levels were related to pre but not post dialysis serum calcium levels and changes to 1-84 PTH levels during dialysis were related to changes in serum calcium levels. 1-84 PTH and iPTH were reduced by haemodialysis, whereas levels of BALP and CTx remained stable post-dialysis. The relationship between BALP and CTx and bone histology requires investigation to determine whether they are more useful markers of bone turnover in this patient group.

Persistent Hypercalcemia After Parathyroidectomy in an Adolescent and Effect of Treatment With Cinacalcet HCl

Background: Hyperparathyroidism is uncommon in adolescence and is more likely to persist after parathyroidectomy than in adults. Cinacalcet HCl is a new calcimimetic that has been used successfully for the treatment of primary and secondary hyperparathyroidism in adults, but its use in adolescents has not been reported.

Case: A 16 year-old male presented with hypercalcemia that had persisted for 1.5 years after parathyroidectomy for primary hyperparathyroidism. Parathyroid hormone (PTH) concentrations were nonsupressed despite a mean (SD) serum calcium concentration of 2.82 (0.06) mmol/L. Treatment with cinacalcet HCl was initiated and a pharmacodynamic profile was obtained for serum calcium, phosphorus, and PTH. Cinacalcet HCl normalized serum calcium. The changes in PTH were assay dependent.

Issues: We use this case conference to review the evaluation of hypercalcemia in adolescents, examine the changes in relevant laboratory results during treatment with cinacalcet HCl, and discuss differences among assays for PTH.

Conclusions: Interpretation of PTH results in patients treated with cinacalcet HCl requires consideration of the pharmacodynamic effects of the drug and the nature of the PTH assay.

Serum osteocalcin and parathyroid hormone in healthy children assessed with two new automated assays

BACKGROUND

The recent introduction of new automated assays needs careful definition of reference values in healthy children. The aim of this study was to determine serum parathyroid hormone (PTH) and osteocalcin in a large group of healthy children according to age.

METHODS

We selected 2,288 healthy children (1,079 girls, 1,209 boys), aged 2-16 years. Serum PTH and osteocalcin were assayed with a two-site immunochemiluminometric assay adapted on an automated analyzer, the Liaison.

RESULTS

Significant differences were found between the mean serum values of PTH and osteocalcin in boys and girls in all age groups (p <0.001). Boys' and girls' PTH values ranged from 3.42-22.30 ng/l and 2.31-24.49 ng/l, respectively; serum osteocalcin ranged from 3.85-17.80 nmol/l in boys and 3.74-17.38 nmol/l in girls.

CONCLUSIONS

The results of this study contribute to the establishment of reference values in healthy children for PTH and osteocalcin assays.

The parathyroid polyhormone hypothesis revisited

The parathyroid polyhormone hypothesis holds that peptides derived from the metabolism of parathyroid hormone (PTH) (so-called C-terminal fragments) are themselves biologically active and that their effects are mediated by a novel 'C-terminal receptor.' The evidence supporting these assertions is extensive but remains inconclusive. This Commentary focuses on in vivo pharmacology studies that provide information relevant to understanding the physiological significance of C-terminal fragments. The more recent studies of this sort provide compelling evidence that the bioactivity of C-terminal fragments is likely to become physiologically relevant in settings of secondary hyperparathyroidism. In this condition, circulating levels of C-terminal fragments greatly exceed those of PTH. There is convincing evidence that the hypocalcemic effect of C-terminal fragments results from direct actions on the skeleton that inhibit bone resorption. On the other hand, there are few if any results of in vivo studies suggesting a role for C-terminal fragments in more physiological settings, at least when parameters associated with systemic calcium homeostasis are assessed.

Inter-method variability in PTH measurement: implication for the care of CKD patients

The National Kidney Foundation/Kidney-Dialysis Outcome Quality Initiative guidelines recommend to maintain the serum intact parathyroid hormone (PTH) concentration between 150 and 300 ng/l in chronic kidney disease (CKD) stage 5 patients. As these limits were derived from studies that used the Allegro intact PTH assay, we aimed to evaluate whether they were applicable to other PTH assays. We compared the PTH concentrations measured with 15 commercial immunoassays in 47 serum pools from dialysis patients, using the Allegro intact PTH assay as the reference. We also evaluated the recovery of graded amounts of synthetic 1-84 and 7-84 PTH added separately to a serum pool. Although the assays were highly correlated, the concentrations differed from one assay to another. The median bias between the tested assays and the Allegro intact PTH assay ranged from -44.9 to 123.0%. When the PTH concentrations were 150 or 300 ng/l with the Allegro intact PTH assay, they ranged with other assays from 83 to 323 ng/l and from 160 to 638 ng/l, respectively. The tested assays recognized 7-84 PTH with various cross-reactivities, whereas a given amount of 1-84 PTH was recovered differently by these assays. We found important inter-method variability in PTH results owing to both antibody specificity and standardization reasons. The unacceptable consequence is that opposite therapeutic attitudes may be reached in a single patient depending on the PTH assay used. We propose to use assay-specific decision limits for CKD patients, or to apply a correcting factor to the PTH results obtained with a given assay.

Fifty years of development in the endocrinology laboratory

The hormone assay laboratory has seen incredible changes over the last 50 years. In this historical review, we describe how the evolution of fundamental concepts in endocrinology and in hormone assay technology have faceted the laboratory as we know it today. The discovery of neurohormones, hormone receptors and the evolution of the concept of free hormones had a very significant impact on our understanding of the mechanisms of hormone action in health and disease and therefore on how physicians currently prescribe endocrine tests. In the analytical field, modern hormone assays rapidly replaced crude colorimetric methods and bioassays. Starting with the pioneering work of Yalow and Berson, hormone assays have gradually evolved through improvements in all aspects of assay design. This is best exemplified by the evolution of thyroid and parathyroid hormone assays. After reviewing some of the limitations of actual hormone immunoassays, we present some reflections on what the future of the hormone laboratory may look like considering all the developments in automation, point-of-care testing, molecular biology and array technologies.

Overproduction and Secretion of a Novel Amino-Terminal Form of Parathyroid Hormone from a Severe Type of Parathyroid Hyperplasia in Uremia

Measurement of bioactive parathyroid hormone (PTH) is essential for optimal management of bone abnormalities in dialysis patients. This can be accomplished by PTH measurements using third-generation PTH assays, which detect more or less of the first six amino acids of the PTH structure. Such assays do not detect non-(1-84) PTH fragments, such as human PTH (7-84), which are recognized by the second-generation PTH assays that use a detection antibody that recognizes an epitope within the 13-34 region of the PTH structure. Therefore, third-generation PTH results are expected to be lower than those that are obtained with second-generation PTH assays. Rare exceptions to this rule have been reported for patients with severe primary hyperparathyroidism or parathyroid cancer. Sera and gland extracts were analyzed from a dialysis patient with high bone turnover disease and with surprising higher PTH levels by a third-generation assay than by a second-generation assay. This finding normalized after the surgical removal of an enlarged gland with a single nodule, an advanced type of nodular hyperplasia. HPLC fractionation of sera and gland extracts revealed the overproduction and secretion of a PTH molecule with an intact amino-terminus structure distinct from (1-84) PTH. This form of PTH was readily detectable by third-generation PTH assays but was poorly reactive in second-generation PTH assays. Therefore, parathyroid glands with advanced uremic nodular hyperplasia may overproduce and secrete a novel, biologically active form of PTH with an intact 1-6 region but a presumably modified 12-18 region required for the detection in second-generation PTH assays.

Practical considerations in PTH testing

New knowledge concerning PTH biology have accumulated during the past few years. The finding that the so-called "intact" PTH assays measure a "non-1-84" PTH fragment in addition to full-length PTH has led to the development of new assays. These new assays, which were initially thought to measure 1-84 PTH only, have been shown to recognize also another PTH species called "amino-PTH". As the various names given to the different assay methods are highly confusing, there is a need for a simplified nomenclature. A simple way would be to identify the older "intact" PTH assays as second-generation assays and the new assays (Whole, CAP, BioIntact) as third-generation assays. Although of considerable potential interest for the comprehension of PTH physiology, the third-generation PTH assays have not yet proved to be superior to the second-generation assays in clinical practice. There is thus currently no recommendation to switch from the second-generation to the third-generation assays in clinical practice, or to use a ratio derived from the concommitent measurement of PTH with both assay-generation. Because second- and third-generation PTH assays are usually highly correlated, significant differences in the clinical information provided by these methods are unlikely. However, our opinion is that more definitive bone biopsy studies in dialyzed patients selected according to their bone- and calcium-related treatment are still needed to reach a consensus. Finally, we have proposed that PTH reference values should be established in healthy subjects with a normal vitamin D status. This supposes that 25OHD is measured in the reference population beforehand, and that the subjects with vitamin D insufficiency are eliminated from the reference group. Although more complicated than the usual way to establish normative data, we have shown that it decreases the upper limit of normal by 25-35%, enhancing thus the diagnostic sensitivity for hyperparathyroidism without a decrease in specificity.