Use of Calcium and Parathyroid Hormone Nomogram to Distinguish Between Atypical Primary Hyperparathyroidism and Normal Patients

Evaluation for primary hyperparathyroidism after radioactive iodine treatment in patients with Graves' disease

BACKGROUND

Retrospective studies are conflicting regarding the risk of primary hyperparathyroidism after radioactive iodine treatment. We hypothesized that primary hyperparathyroidism rates are greater after radioactive iodine than after thyroidectomy and that patients with hypercalcemia after treatment for Graves' disease are not adequately evaluated for primary hyperparathyroidism, contributing to underdiagnosis of radioactive iodine-associated primary hyperparathyroidism.

METHODS

This retrospective review considers patients undergoing radioactive iodine or thyroidectomy for Graves' disease at a tertiary referral center between January 1, 2000, and January 31, 2022. Patients were identified using a hospital-based cohort discovery tool; exclusions included history of head/neck radiation, primary hyperparathyroidism/parathyroidectomy, renal dysfunction diagnoses, or treatment with both radioactive iodine and thyroidectomy. Patients with an elevated calcium (>10.2 mg/dL) level measured after treatment were classified as "incomplete workup" (no parathyroid hormone), "likely primary hyperparathyroidism" (parathyroid hormone >40 pg/dL), or "unlikely primary hyperparathyroidism" (parathyroid hormone <40 pg/dL).

RESULTS

Of 900 patients, 468 (52%) had been treated with radioactive iodine and 432 (48%) with thyroidectomy. At a median follow-up of 9.39 years (interquartile range, 5.12-13.25), 25% (n = 224) of patients did not have a serum calcium measured and 52 (8%, n = 676) patients had an elevated calcium level. Hypercalcemia was more common after radioactive iodine (10%) than thyroidectomy (6%, P = .061). Of patients with hypercalcemia, 33 (63%) were "incomplete workup," 5 (10%) were "likely primary hyperparathyroidism," and 14 (27%) were "unlikely primary hyperparathyroidism." There was no difference in primary hyperparathyroidism workup rates between patients treated with radioactive iodine (n = 23) and thyroidectomy (n = 10, P = .389).

CONCLUSIONS

Patients treated with radioactive iodine for Graves' disease may experience an elevated rate of hypercalcemia, but the majority are not adequately evaluated for primary hyperparathyroidism. Patients with a history of radioactive iodine should undergo regular calcium screening and, if elevated, appropriate workup for primary hyperparathyroidism.

Primary hyperparathyroidism

Primary hyperparathyroidism (PHPT), the most common cause of hypercalcemia, is most often identified in postmenopausal women with hypercalcemia and parathyroid hormone (PTH) levels that are either frankly elevated or inappropriately normal. The clinical presentation of PHPT includes three phenotypes: target organ involvement of the renal and skeletal systems; mild asymptomatic hypercalcemia; and more recently, high PTH levels in the context of persistently normal albumin-corrected and ionized serum calcium values. The factors that determine which of these three clinical presentations is more likely to predominate in a given country include the extent to which biochemical screening is employed, the prevalence of vitamin D deficiency, and whether a medical center or practitioner tends to routinely measure PTH levels in the evaluation of low bone density or frank osteoporosis. When biochemical screening is common, asymptomatic primary hyperparathyroidism is the most likely form of the disease. In countries where vitamin D deficiency is prevalent and biochemical screening is not a feature of the health care system, symptomatic disease with skeletal abnormalities is likely to predominate. Finally, when PTH levels are part of the evaluation for low bone mass, the normocalcemic variant is seen. Guidelines for surgical removal of hyperfunctioning parathyroid tissue apply to all three clinical forms of the disease. If guidelines for surgery are not met, parathyroidectomy can also be an appropriate option if there are no medical contraindications to surgery. In settings where either the serum calcium or bone mineral density is of concern, and surgery is not an option, pharmacological approaches are available and effective. Referencing in this article the most current published articles, we review the different presentations of PHPT, with particular emphasis on recent advances in our understanding of target organ involvement and management.

Phenotypes of primary hyperparathyroidism: Does parathyroidectomy improve clinical outcomes for all?

BACKGROUND

Primary hyperparathyroidism consists of 3 biochemical phenotypes: classic, normocalcemic, and normohormonal primary hyperparathyroidism. The clinical outcomes of patients with normocalcemic primary hyperparathyroidism and normohormonal primary hyperparathyroidism are not well described.

METHOD

A retrospective review of patients who underwent parathyroidectomy at a single institution was performed. A logistical regression analysis of postoperative nephrolithiasis and highest percentage change in dual-energy x-ray absorptiometry scan comparison using Kruskal-Wallis test and Cox proportional hazard analysis of recurrence-free survival were performed.

RESULTS

A total of 421 patients were included (340 classic, 39 normocalcemic primary hyperparathyroidism, 42 normohormonal primary hyperparathyroidism). Median follow-up was 8.8 months (range 0-126). Higher rates of multigland disease were seen in normocalcemic primary hyperparathyroidism (64.1%) and normohormonal primary hyperparathyroidism (56.1%) compared to the classic (25.8%), P < .001. There were no differences in postoperative complications. The largest percentage increases in bone mineral density at the first postoperative dual-energy x-ray absorptiometry scan were higher for classic (mean ± SD, 6.4 ± 9.1) and normocalcemic primary hyperparathyroidism (4.8 ± 11.9) compared to normohormonal primary hyperparathyroidism, which remained stable (0.2 ± 14.2). Normocalcemic primary hyperparathyroidism were more likely to experience nephrolithiasis postoperatively, 6/13 (46.2%) compared to 11/68 (16.2%) classic, and 2/13 (15.4%) normohormonal primary hyperparathyroidism, P = .0429. Normocalcemic primary hyperparathyroidism was the only univariate predictor of postoperative nephrolithiasis recurrence (odds ratio [95% confidence interval] 4.44 [1.25-15.77], P = .029). Normocalcemic primary hyperparathyroidism was significantly associated with persistent disease with 6/32 (18.8%) compared to 1/36 (2.8%) and 3/252 (1.2%) in normohormonal primary hyperparathyroidism and classic (P < .001).

CONCLUSION

Three phenotypes of primary hyperparathyroidism are distinct clinical entities. Normocalcemic primary hyperparathyroidism had higher incidence of persistent disease and postoperative nephrolithiasis but demonstrated improvements in postoperative bone density. These data should inform preoperative discussions with patients with normocalcemic primary hyperparathyroidism and normohormonal primary hyperparathyroidism regarding postoperative expectations.

Hypercalcemia with a parathyroid hormone level of ≤50 pg/mL: Is this primary hyperparathyroidism?

BACKGROUND

Primary hyperparathyroidism is characterized by hypercalcemia with inappropriately normal or elevated parathyroid hormone. However, the absolute parathyroid hormone value that is defined as inappropriately normal is unclear. We reviewed our experience with parathyroidectomy in patients with hypercalcemia and parathyroid hormone of ≤50.0 pg/mL (normal range 12.0-88.0 pg/mL).

METHODS

A total of 2,349 patients underwent parathyroidectomy for primary hyperparathyroidism between 2000 and 2021. Of these, 149 patients had preoperative parathyroid hormone ≤50.0 pg/mL (parathyroid hormone ≤50). The biology and outcomes were compared to patients with parathyroid hormone >50.0 pg/mL (parathyroid hormone >50).

RESULTS

Of the parathyroid hormone ≤50 patients, the median parathyroid hormone was 40.0 pg/mL (range 11.6-50.0 pg/mL). All patients were found to have abnormal hypercellular parathyroid glands with a cure rate of 96.7%. When compared to the parathyroid hormone >50 group, the parathyroid hormone ≤50 group was younger (56 ± 15 vs 60 ± 14 years, P < .001) with a lower body mass index (28.7 ± 7.0 kg/m² vs 31.2 ± 7.9 kg/m², P < .001), higher rate of fatigue (73.2% vs 63.0%, P = .033), and higher rate of multiglandular disease (58.9% vs 31.9%, P < .001). There was no difference between the groups with respect to patient demographics, rate of previous parathyroidectomy, surgical cure, or postoperative complications (persistent or recurrent hyperparathyroidism and hypocalcemia).

CONCLUSION

Patients with hypercalcemia and parathyroid hormone ≤50 pg/mL have a similar clinical presentation and rate of surgical cure to other primary hyperparathyroidism patients and should be considered for parathyroidectomy. Most of these patients have multiglandular disease and thus should be considered for bilateral parathyroid exploration.

Classical and Nonclassical Manifestations of Primary Hyperparathyroidism

This narrative review summarizes data on classical and nonclassical manifestations of primary hyperparathyroidism (PHPT). It is based on a rigorous literature search, inclusive of a Medline search for systematic reviews from 1940 to December 2020, coupled with a targeted search for original publications, covering four databases, from January 2013-December 2020, and relevant articles from authors' libraries. We present the most recent information, identify knowledge gaps, and suggest a research agenda. The shift in the presentation of PHPT from a predominantly symptomatic to an asymptomatic disease, with its varied manifestations, has presented several challenges. Subclinical nephrolithiasis and vertebral fractures are common in patients with asymptomatic disease. The natural history of asymptomatic PHPT with no end organ damage at diagnosis is unclear. Some observational and cross-sectional studies continue to show associations between PHPT and cardiovascular and neuropsychological abnormalities, among the different disease phenotypes. Their causal relationship is uncertain. Limited new data are available on the natural history of skeletal, renal, cardiovascular, neuropsychological, and neuromuscular manifestations and quality of life. Normocalcemic PHPT (NPHPT) is often diagnosed without the fulfillment of rigorous criteria. Randomized clinical trials have not demonstrated a consistent long-term benefit of parathyroidectomy (PTX) versus observation on nonclassical manifestations. We propose further refining the definition of asymptomatic disease, into two phenotypes: one without and one with evidence of target organ involvement, upon the standard evaluation detailed in our recommendations. Each of these phenotypes can present with or without non-classical manifestations. We propose multiple albumin-adjusted serum calcium determinations (albumin-adjusted and ionized) and exclusion of all secondary causes of high parathyroid hormone (PTH) when establishing the diagnosis of NPHPT. Refining the definition of asymptomatic disease into the phenotypes proposed will afford insights into their natural history and response to interventions. This would also pave the way for the development of evidence-based guidance and recommendations. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Comparisons Between Normocalcemic Primary Hyperparathyroidism and Typical Primary Hyperparathyroidism

BACKGROUND

Normocalcemic primary hyperparathyroidism (NPHPT) was first described in 2008. It is defined as consistently elevated serum parathyroid hormone (PTH) levels with normal serum calcium (sCa) concentration, after excluding secondary causes of PTH elevation. However, the exact definition and management strategy for NPHPT remain controversial. We retrospectively investigated the clinicopathological features and short-term outcomes of NPHPT patients.

METHODS

A total of 280 patients who were surgically indicated for primary hyperparathyroidism (PHPT) at the Yonsei Severance Medical Center between 2015 and 2019 were included. Patients were classified according to preoperative PTH, corrected sCa, and ionized calcium (iCa) levels as follows: typical primary hyperparathyroidism (TPHPT, elevated PTH, sCa, and iCa, n = 158) and NPHPT (elevated PTH, normal sCa, n = 122).

RESULTS

NPHPT was commonly seen in younger individuals (aged < 50 years, P = 0.025); nephrolithiasis and bone fractures were common. Preoperative PTH level was higher in the TPHPT group (P < 0.001). The NPHPT group had higher numbers of multiple parathyroid lesions (P = 0.004) that were smaller (P = 0.011). NPHPT patients were further divided into two subgroups according to iCa levels: the elevated (n = 95) and normal iCa (n = 27) groups. There was no significant difference between the two subgroups regarding symptoms and multiplicity of lesions.

CONCLUSION

We found that NPHPT may be a heterogeneous disease entity of PHPT with high rates of multi-gland disease, which appears to be biochemically milder but symptomatic. Intraoperative PTH monitoring might help increase the surgery success rate. Moreover, the short-term outcomes of NPHPT after surgery did not differ from that of TPHPT.

Primary hyperparathyroidism: Disease of diverse genetic, symptomatic, and biochemical phenotypes

Genetic, symptomatic, and biochemical heterogeneity of patients with primary hyperparathyroidism (PHPT) has become apparent in recent years. An in-depth, evidence-based review of the phenotypes of PHPT was conducted. This review was intended to provide the resulting information to surgeons who operate on patients with hyperparathyroidism. This review revealed that the once relatively clear distinction between familial and sporadic PHPT has become more challenging by the finding of various germline mutations in patients with seemingly sporadic PHPT. On the one hand, the genetic and clinical characteristics of some syndromes in which PHPT is an important component are now better understood. On the other hand, knowledge is emerging about novel syndromes, such as the rare multiple endocrine neoplasia type IV (MEN4), in which PHPT occurs frequently. It also revealed that, currently, the classical array of symptoms of PHPT is seen rarely upon initial presentation for evaluation. More common are nonspecific, nonclassical symptoms and signs of PHPT. In areas of the world where serum calcium levels are checked routinely, most patients today are "asymptomatic" and they are diagnosed after an incidental finding of hypercalcemia; however, some of them have subclinical involvement of bones and kidneys, which is demonstrated on radiographs, ultrasound, and modern imaging techniques. Last, the review points out that there are three distinct biochemical phenotypes of PHPT. The classical phenotype in which calcium and parathyroid hormone levels are both elevated, and other disease presentations in which the serum levels of calcium or intact parathyroid hormone are normal. Today several, distinct phenotypes of the disease can be identified, and they have implications in the diagnostic evaluation and treatment of patients, as well as possible screening of relatives.

The Significance of Histologically "Large Normal" Parathyroid Glands in Primary Hyperparathyroidism

INTRODUCTION

We investigated outcomes in a cohort of patients with a biochemical diagnosis of primary hyperparathyroidism (pHPT) undergoing surgery for asymptomatic disease or target organ damage, where a focussed or four-gland operation was undertaken and the histopathology only reported a "large normal" parathyroid gland (LNP).

METHODS AND MATERIALS

Patients subjected to a parathyroidectomy for pHPT between 2012 and 2018 with a pathology of LNP were included. Patients with fat depletion or additional histological features of adenoma or hyperplasia in any of the resected glands were excluded. A control group was formed from 50 consecutive patients with the histological finding of adenoma or hyperplasia during the same study period. The primary outcome was biochemical normalisation of pHPT at 1-2 weeks and after 6 months post-operatively.

RESULTS

Forty-eight LNP patients (2% of all parathyroidectomies) were included in the study group with 50 matched controls. LNP patients had a lower biochemical cure rate (81% vs. 98% P < 0.05) and a higher risk of recurrence (10% vs. 0%, P = 0.06). LNP patients had a milder form of pHPT (Ca²⁺ 2.63 vs. 2.68 P < 0.05) with a smaller PTH and Ca²⁺ drop post-operatively. For LNP patients with failure, a definite additional cause of pHPT was found in only two patients.

CONCLUSION

This study highlights a controversial area in pHPT and reports LNP as a cause of pHPT. The biochemical analysis of this LNP group supports a benefit in resection in the setting of pHPT, although the risk of failure (persistence/recurrence) is higher than those with adenoma or hyperplasia. Stricter post-operative follow-up of LNP patients should be considered.

Biochemical Profile Affects IOPTH Kinetics and Cure Rate in Primary Hyperparathyroidism

BACKGROUND

Recently, normocalcemic (NC) and normohormonal (NH) variants of primary hyperparathyroidism (pHPT) have been described, with distinct biochemical profiles from the typical high serum calcium and parathyroid hormone (PTH) levels. Here, we investigate whether biochemical profile affects cure rate, as well as the kinetics of intraoperative PTH (IOPTH).

METHODS

This is a single-center, retrospective study of pHPT patients undergoing parathyroidectomy. Patients were grouped based on preoperative calcium and PTH levels into typical, NC (normal calcium, elevated PTH, no evidence of secondary hyperparathyroidism), and NH (elevated calcium, unsuppressed PTH) biochemical profiles. All patients had IOPTH monitoring and ≥6-month post-op serum studies to confirm surgical cure. Patient variables were analyzed with Kruskal-Wallis test and Chi-square analysis. IOPTH kinetic curves were analyzed using a linear mixed model.

RESULTS

From June 2006 to October 2014, 646 patients met inclusion criteria. Biochemical profile was typical in 460 patients (71%), NC in 101 (16%), and NH in 85 (13%). IOPTH levels were higher at all time points in typical patients, p < 0.001. Surgical cure rates were significantly lower for NC patients (90.1%) than for typical (98.5%) or NH patients (97.7%), p < 0.001, although a stricter criteria for cure was used in this group (normal calcium AND normal PTH). In a multivariable linear mixed model, NC patients had a significantly slower rate of IOPTH decline (p < 0.001 at 10 min).

CONCLUSIONS

Here, we better characterize the atypical variants of pHPT. Using a stricter definition of cure in the NC variant, these patients have a lower surgical cure rate than typical or NH variants in pHPT. The IOPTH curve is affected by biochemical profile, with both NC and NH patients having lower absolute values and NC patients having a slower decline.

Screening for primary hyperparathyroidism in a tertiary stone clinic, a useful endeavor

INTRODUCTION AND OBJECTIVES

Primary hyperparathyroidism (1HPT) is associated with the risk of developing kidney stones. Our objective was to determine the prevalence of 1HPT amongst SF evaluated at a tertiary stone clinic and determine if it is cost-effective to screen for this condition.

METHODS

We retrospectively reviewed 742 adult SF seen by a single urologic surgeon from 2012 to 2017 all of who were screened for 1HPT with an intact serum PTH (iPTH) and calcium. The diagnosis of 1HPT was based on the presence of hypercalcemia with an inappropriately elevated iPTH or a high normal serum calcium and an inappropriately elevated iPTH. The diagnosis was confirmed by surgical neck exploration. Published cost data and stone recurrence rates were utilized to create a cost-effectiveness decision tree.

RESULTS OBTAINED

Fifty-three (7.1%) were diagnosed with 1HPT. 15 (28%) had hypercalcemia and inappropriately elevated iPTH, 38 (72%) had high normal serum calcium levels and inappropriately elevated iPTH. The potential diagnosis was ignored/missed by primary care physicians in 9 (17.0%) based on a review of prior lab results. Cost modeling was undertaken for 5, 10, 15, and 20-year intervals after screening. Based on our prevalence data, historical risks for recurrence and published cost data for stone treatments, cost savings in screening are realized at 10 years.

CONCLUSION

These results support screening for primary hyperparathyroidism in patients evaluated in a tertiary referral setting.

Primary Hyperparathyroidism

Background

Primary hyperparathyroidism (PHPT), the most common cause of hypercalcemia, is most often identified in postmenopausal women. The clinical presentation of PHPT has evolved over the past 40 years to include three distinct clinical phenotypes, each of which has been studied in detail and has led to evolving concepts about target organ involvement, natural history, and management.

Methods

In the present review, I provide an evidence-based summary of this disorder as it has been studied worldwide, citing key concepts and data that have helped to shape our concepts about this disease.

Results

PHPT is now recognized to include three clinical phenotypes: overt target organ involvement, mild asymptomatic hypercalcemia, and high PTH levels with persistently normal albumin-corrected and ionized serum calcium values. The factors that determine which of these clinical presentations is more likely to predominate in a given country include the extent to which biochemical screening is used, vitamin D deficiency is present, and whether parathyroid hormone levels are routinely measured in the evaluation of low bone density or frank osteoporosis. Guidelines for parathyroidectomy apply to all three clinical forms of the disease. If surgical guidelines are not met, parathyroidectomy can also be an appropriate option if no medical contraindications are present. If either the serum calcium or bone mineral density is of concern and surgery is not an option, pharmacological approaches are available and effective.

Conclusions

Advances in our knowledge of PHPT have guided new concepts in diagnosis and management.

Primary hyperparathyroidism

Primary hyperparathyroidism (PHPT), the most common cause of hypercalcemia, is most often identified in postmenopausal women with hypercalcemia and parathyroid hormone (PTH) levels that are either frankly elevated or inappropriately normal. The clinical presentation of PHPT includes three phenotypes: target organ involvement of the renal and skeletal systems; mild asymptomatic hypercalcemia; and more recently, high PTH levels in the context of persistently normal albumin-corrected and ionized serum calcium values. The factors that determine which of these three clinical presentations is more likely to predominate in a given country include the extent to which biochemical screening is employed, the prevalence of vitamin D deficiency, and whether a medical center or practitioner tends to routinely measure PTH levels in the evaluation of low bone density or frank osteoporosis. When biochemical screening is common, asymptomatic primary hyperparathyroidism is the most likely form of the disease. In countries where vitamin D deficiency is prevalent and biochemical screening is not a feature of the health care system, symptomatic disease with skeletal abnormalities is likely to predominate. Finally, when PTH levels are part of the evaluation for low bone mass, the normocalcemic variant is seen. Guidelines for surgical removal of hyperfunctioning parathyroid tissue apply to all three clinical forms of the disease. If guidelines for surgery are not met, parathyroidectomy can also be an appropriate option if there are no medical contraindications to surgery. In settings where either the serum calcium or bone mineral density is of concern, and surgery is not an option, pharmacological approaches are available and effective. Referencing in this article the most current published articles, we review the different presentations of PHPT, with particular emphasis on recent advances in our understanding of target organ involvement and management.

Classic Primary Hyperparathyroidism Versus Normocalcemic and Normohormonal Variants: Do They Really Differ?

BACKGROUND

Normocalcemic (NCpHPT) and normohormonal (NHpHPT) variants have been recognized primary hyperparathyroidism entities that pose serious challenges. We sought to define the differences among them in a series of surgically treated patients.

PATIENTS AND METHODS

Between 2011 and 2015, 149 patients were enrolled into three groups: CpHPT (Ca > 10.2 mg/dL, PTH > 65 pg/mL), NCpHPT (normal Ca, PTH > 65 pg/mL) and NHpHPT (Ca > 10.2 mg/dL, normal PTH). Descriptive statistics and inter-group differences were computed, whereas multiple logistic/linear regression tests were used for further analysis.

RESULTS

Of these patients 125 were female and 24 male, mean age 56.3 years (range 8-83). A total of 115 (77.2%) patients presented with CpHPT, 23 (15.4%) with NCpHPT and 11 (7.4%) with NHpHPT. MGD was found in 25 (16.8%) patients and SGD in 124 (83.2%); multivariate analysis failed to reveal statistically significant association of MGD with any pHPT variant (CpHPT 16.5% vs NCpHPT 21.7% vs NHpHPT 9.1%, p = 0.726). Conversely, NCpHPT patients exhibited statistically significant smaller adenoma weight (p = 0.023). Moreover, U/S in these patients had smaller positive predictive value (p = 0.278), whereas concordance between U/S and MIBI was also lower (p = 0.669). The utility of MIBI and U/S differed significantly (p < 0.001); more frequent use of U/S was observed for all groups. However, their predictive values did not differ significantly (p = 0.832).

CONCLUSIONS

NCpHPT is more similar than different to CpHPT. NCpHPT constitutes the most challenging entity: it is associated with smaller adenoma weight, whereas U/S exhibited lower positive predictive value and lower concordance rate with MIBI. A trend for higher MGD presence in this group of patients was observed, though without statistical significance.

Timely Evaluation and Management of Primary Hyperparathyroidism in Patients With Kidney Stones

BACKGROUND

Nephrolithiasis is a classic, treatable manifestation of primary hyperparathyroidism (PHPT). We examined predictors of kidney stone formation in PHPT patients and determined how efficiently the diagnosis of PHPT is made in patients whose initial presentation is with stones.

MATERIALS AND METHODS

We performed a retrospective analysis of surgically treated PHPT patients, comparing 247 patients who were kidney stone formers and 1047 patients with no kidney stones. We also analyzed 51 stone-forming patients whose stone evaluation and treatment were completed within our health system before PHPT diagnosis.

RESULTS

Stone-forming patients had higher 24-h urinary calcium (342 versus 304 mg/d, P = 0.005), higher alkaline phosphatase (92 versus 85 IU/L, P = 0.012), and were more likely to be normocalcemic (26.6% versus 16.9%, P = 0.001). Surprisingly, 47.3% of stone formers had normal urinary calcium levels (<300 mg/d). Of the 51 stone-forming patients treated at our institution, serum calcium was measured within 6 mo of stone diagnosis in 37 (72.5%) patients. Only 16 (31.4%) of these patients had elevated calcium levels, and only 10 (62.5%) of these 16 had a serum parathyroid hormone ordered within the following 3 mo. These patients had a significantly shorter time from their first stone to surgical treatment compared to other patients (median 8.5 versus 49.1 mo, P = 0.001).

CONCLUSIONS

Elevated serum and urinary calcium levels are not evaluated in the majority of PHPT patients presenting with kidney stones. In nephrolithiasis patients, provider consideration of PHPT with prompt serum calcium and parathyroid hormone evaluation significantly reduces time to treatment.

Familial Hypocalciuric Hypercalcemia as an Atypical Form of Primary Hyperparathyroidism

Familial hypocalciuric hypercalcemia (FHH) causes lifelong hypercalcemia with features that overlap with typical primary hyperparathyroidism (PHPT). The incompleteness of this overlap has led to divergent nomenclatures for FHH. I compare two nomenclatures. One sets FHH as an entity distinct from PHPT. The other groups FHH with PHPT but conditions FHH as atypical PHPT. I analyzed selected articles about calcium-sensing receptors, FHH, PHPT, CASR, GNA11, and AP2S1. FHH usually results from a heterozygous germline inactivating mutation of the CASR, and less frequently from mutation of GNA11 or AP2S1. The CASR encodes the calcium-sensing receptors. These are highly expressed on parathyroid cells, where they sense serum calcium concentration and regulate suppression of PTH secretion by serum calcium. Their mutated expression in the kidney in FHH causes increased renal tubular reabsorption of calcium (hypocalciuria). Many FHH features are shared with PHPT and thus support FHH as a form of PHPT. These include a driver mutation expressed mainly in the parathyroid cells. The mutation causes a parathyroid cell insensitivity to extracellular calcium in vivo and in vitro, a right-shift of the set point for suppression of PTH secretion by calcium. Serum PTH is normal or mildly elevated; ie, it is not appropriately suppressed by hypercalcemia. Total parathyroidectomy causes hypoparathyroidism and durable remission of hypercalcemia. Some other features are not shared with PHPT and could support FHH as a distinct entity. These include onset of hypercalcemia in the first week of life, frequent persistence of hypercalcemia after subtotal parathyroidectomy, and hypocalciuria. The features supporting FHH as a form of PHPT are stronger than those favoring FHH as a distinct entity. Classifying FHH as an atypical form of PHPT represents compact nomenclature and supports current concepts of pathophysiology of FHH and PHPT. Published 2017. This article is a U.S. Government work and is in the public domain in the USA.