Case Presentation: Hypercalcemia in a Young Woman

Samina Afreen, MD
Previous Fellow, Division of
Endocrinology and Metabolism,
University of Pittsburgh School of Medicine

Helena Levitt, MD
Clinical Assistant Professor of Medicine,
Division of Endocrinology and Metabolism,
University of Pittsburgh School of Medicine

Case Presentation

A 37-year-old Caucasian female presented to the UPMC Endocrine Genetics Clinic in 2019 for evaluation and management of hypercalcemia and hyperparathyroidism. She had previously undergone parathyroid surgery at an outside institution in 2013 for presumed primary hyperparathy-roidism (pHPT). Since then, her serum calcium remained elevated in the range of 10.4–11.1 mg/dL with concomitant nonsuppressed parathyroid hormone (PTH) levels ranging from 66–178 pg/mL.

The patient initially was noted to have hypercalcemia on routine laboratory evaluation in 2008 when she was 26 years old. She had no known prior history of hypercalcemia, hyperparathyroidism, nephrolithiasis, fracture, or other condition associated with abnormal serum calcium levels. Likewise, she had no known family history of hypercalcemia or hyperpara-thyroidism. Her initial serum calcium was 10.9 mg/dL, and a confirmatory repeat albumin-corrected serum calcium was 11.0 mg/dL. A concurrent serum PTH was 170 pg/mL. Serum 25-hydroxy vitamin D was within the normal range. Additional oral vitamin D supplementation did not suppress the serum PTH level. Bone mineral densitometry (BMD) by dual x-ray absorptiometry (DXA) was normal. The 24-hour urinary calcium to creatinine clearance ratio (UCCR) was 0.01 on two occasions. Based on the above information, it was felt that both pHPT and familial hypocalciuric hypercalcemia (FHH) remained possible diagnoses, so the patient was managed conservatively with annual clinical re-evaluation.

After five years, the patient ultimately decided to undergo parathyroid exploration in 2013. During this parathyroid exploration, a minimally enlarged left inferior parathyroid gland was identified and resected. However, her intraoperative serum PTH level did not decrease significantly (178 pg/mL to 122 pg/mL). Subsequent calcium and PTH levels at two weeks and three months postoperatively remained elevated, providing evidence supporting FHH as a potential diagnosis. She was again managed conservatively with annual clinical re-evaluation until 2018 when she again presented for surgical consultation. Repeat-operation was deferred, and instead, her surgeon recommended genetic testing for calcium-sensing receptor (CASR) mutations. The results revealed a variant of undetermined significance (VUS) in her CASR gene. Her surgeon then referred her to the UPMC Endocrine Genetics Clinic for further evaluation and management in early 2019.

The UPMC Endocrine Genetics Clinic consists of a team of endocrinologists, endocrine surgeons, and clinical geneticists with specialized expertise in genetic endocrine disorders. Clinical history confirmed the key elements noted above. History and physical exam likewise confirmed that the patient was generally in excellent health other than the elevated serum calcium and PTH levels. She had no personal or family history of any conditions associated with abnormal serum calcium or PTH. She had no other significant medical history. Family history was notable for lung cancer in her paternal grandfather, leukemia in her maternal grandfather, and stomach cancer in her paternal grandmother. She had no allergies. She was not taking any medications or over-the-counter supplements. Laboratory data revealed elevated serum calcium of 10.7 mg/dL and PTH of 120 pg/mL. Other electrolytes, including magnesium and phosphorus, were normal. Serum 25-hydroxy vitamin D also was normal. The UCCR was 0.015.

After clinical evaluation of the patient, the endocrine genetics team decided to pursue additional genetic testing, including testing for FHH1 (CASR), FHH2 (GNA11), and FHH3 (AP2S1), as well as additional genes associated with familial hyperparathyroidism. Subsequent results were negative for mutations in the CASR, GNA11, AP2S1, and CDC73 genes, as well as the CDKN1B, MEN-1, RET, and GCM2 genes. In addition, the prior 2013 CASR VUS results were revised to indicate that this VUS was predicted to be nonpathogenic.

With a UCCR of 0.01-0.02 and persistent hypercalcemia following parathyroid surgery, FHH remains a possible explanation for the patient’s PTH-mediated hypercalcemia. However, genetic testing results for CASR, AP2S1, and GNA11 mutations failed to provide confirmatory evidence for this diagnosis. The patient was encouraged to have her first-degree family members tested for hypercalcemia, because identification of additional cases of familial hypercalcemia would support a diagnosis of FHH. Thus far, normal serum calcium levels have been confirmed in her three children.

Discussion

This case demonstrates many of the diagnostic challenges in differentiating between primary hyperparathyroidism (pHPT) and familial hypocalciuric hypercalcemia (FHH). These diagnoses have different clinical courses and require different treatments. pHPT can be associated with significant morbidity, whereas FHH generally has a more benign clinical course. Since treatment of FHH can cause harm, it is essential to rule out this diagnosis before pursuing medical or surgical therapy for hypercalcemia. A diagnosis of FHH also has implications for affected family members. Surgery is the definitive treatment for pHPT but should be avoided with FHH. Clinical suspicion for FHH should be higher in asymptomatic patients with a low 24-hour UCCR who also are diagnosed with parathyroid-dependent hypercalcemia at a young age, in patients with a family history of hypercalcemia,

or patients with multiglandular disease. Clinical and biochemical evidence may be insufficient to differentiate between pHPT and FHH definitively. Indeed, FHH, which represents ~2 percent of pHPT cases, is diagnosed in up to 23 percent of cases that failed to respond to parathyroid surgery. Recent advances in understanding the genetic causes of FHH have improved the diagnosis and treatment of FHH. This article reviews the diagnostic challenges and use of genetic testing in differentiating between pHPT and FHH. 

Serum Biochemical Abnormalities

Because of the significant overlap of serum parameters between FHH and pHPT, serum biochemical studies are often inadequate to differentiate between these diagnoses. In pHPT, calcium levels can be mildly to severely elevated and may progressively worsen over time. Serum phosphorus levels tend to be in the low-normal range but can be low in approximately one-third of cases. Serum 25-hydroxy vitamin D levels are typically normal or low-normal, in part due to PTH-driven conversion of 25-hydroxy vitamin D levels to 1,25-dihydroxy vitamin D. Correspondingly, 1,25-dihydroxy vitamin D levels tend to be high or high-normal. In FHH, calcium is mildly and stably elevated throughout life (generally below 12 mg/dL). PTH levels are normal in approximately 80 percent of patients and mildly elevated in the other 20 percent. Serum phosphorus levels are often reduced. Serum 1,25-dihydroxy vitamin D levels are normal or elevated. In addition, a history of hypercalcemia at a young age is helpful in differentiating between these diagnoses.

Urine Calcium to Creatinine Ratio (UCCR)

The UCCR [calculated as a (24-hour urine calcium in mg x serum creatinine in mg/dL)/ (serum calcium in mg/dL x 24-hour urine creatinine in mg)] can help to differentiate between FHH and pHPT. Different UCCR cutoff criteria have been proposed. Generally, a UCCR of less than 0.01 is most consistent with FHH, whereas a UCCR of greater than 0.02 is most consistent with pHPT. However, the utility of the UCCR is not without limitations. Between 63 to 68 percent of patients with confirmed pHPT have UCCRs of less than 0.02.1 Likewise, up to 35 percent of patients with confirmed FHH have UCCRs of greater than 0.01.¹

Several factors can influence urinary calcium excretion and, consequently, the UCCR. Vitamin D deficiency (25-hydroxy vitamin D < 10 ng/mL) is associated with 27 percent higher PTH hypersecretion, 26 percent lower impairment of urinary calcium excretion, and reduced sensitivity of the UCCR in diagnosing pHPT.² Thiazide diuretics, estrogen, and potassium citrate can decrease urinary calcium excretion. Conversely, many pharmacological agents can increase urinary calcium excretion, including drugs that contain calcium (antacids and calcium supplements), diuretics (spironolactone and furosemide), androgens, growth hormone, systemic corticosteroids, and acetazolamide.³ In addition, ethnic differences in urinary calcium excretion have been noted. For example, African Americans generally have lower urinary calcium excretion than Caucasians.⁴ A study published in 2018, conducted with a subject size of 1,000 patients, suggested that UCCR was nondiscriminatory for all biochemical presentations, subtle and more severe.¹

Thus, the Proceedings of the Fourth International Workshop for Diagnosis of Asymptomatic Primary Hyperparathy-roidism recommends that if UCCR is 0.01–0.02 and 25-hydroxy vitamin D is greater than 20 ng/mL, with a normal eGFR of greater than 60 mL/min, then genetic testing for FHH should be considered (see Page 10).⁵ 

Dual X-ray Absorptiometry (DXA)

A DXA scan of a patient presenting with pHPT generally shows reduced BMD of the distal third of the forearm, a site enriched in cortical bone, whereas the lumbar spine, a site enriched in cancellous bone, is relatively preserved. However, approximately 15 percent of patients with pHPT present with an atypical BMD profile, characterized by vertebral osteopenia or osteoporosis. Occasionally, patients with pHPT may show reduced BMD at all sites. In contrast, FHH usually is not associated with BMD abnormalities, though abnormalities may be noted. In addition, FHH3, in particular, is associated with reduced BMD along with cognitive deficits and/or behavioral disturbances in children.⁶

Genetic Testing

FHH is a well-established heritable disorder of serum calcium homeostasis. However, only recently has a vast array of genetic contributions to FHH been delineated. FHH was initially identified to be caused by an inactivating mutation in the calcium-sensing receptor (CASR) gene, a G-protein-coupled calcium-sensing receptor localized on chromosome 3q13.3-21.3. CASR mutations define FHH type 1 (FHH1). However, up to 30 percent of cases with a typical FHH phenotype do not harbor CASR mutations. In these cases, analysis for loci linked to hyper-calcemia on chromosome 19p or 19q.13 revealed additional causative mutations in G Protein Subunit alpha 11 (GNA11, FHH-2)⁷ and Adaptor Related Protein Complex 2 Subunit Sigma 1 (AP2S1, FHH-3),⁸ respectively. 

GNA11 is a gene that regulates CASR activity. A loss-of-function mutation in GNA11 reduces signaling through CASR and causes hypercalcemia. In contrast, a gain-of-function mutation in GNA11 increases signaling through CASR and causes hypocalcemia. The latter is designated as autosomal dominant hypocalcemia type 2 to distinguish it from type 1, which is due to gain-of-function of the CASR.

AP2S1 is a gene that regulates CASR cellular trafficking. Loss-of-function mutations in GNA11 reduce signaling through CASR and cause hypercalcemia. A hotspot missense mutation in codon 15 is one of the causes of FHH3.⁹ AP2S1 may be involved in psychiatric diseases and depression. Due to this possibility, patients with isolated familial hyperparathyroidism and a phenotype compatible with FHH who have learning disabilities and/or psychiatric disorders should be genetically tested for AP2S1 exon 2 mutations before proceeding to other genetic tests. This set of testing will gather results more quickly than whole CASR sequencing while also being more cost-efficient. Patients who do not present with these comorbid diagnoses should have an evaluation of CASR as an initial genetic test.⁸ 

Sporadically occurring new mutations causing FHH are seen in 15 to 30 percent of new index cases of FHH1.¹⁰ Thirty percent of typical FHH cases are negative for a CASR mutation. In these cases, further analyses may identify mutations in GNA11 (FHH2) or AP2S1 (FHH3). FHH2 mutations, found in ~20 percent of FHH patients with negative CASR testing, are more common than FHH3, which is found in less than 5 percent of patients with FHH and normal CASR gene sequencing. Acquired auto-antibodies that block the interaction of extracellular calcium with CASR have been found in patients presenting with the FHH phenotype without CASR mutations.¹⁰

Genetic testing for Multiple Endocrine Neoplasia type 1 (MEN-1) should be considered in patients with pHPT who are below the age of 30 or at any age in patients presenting with multiglandular parathyroid disease.¹¹ Patients who meet the criteria for a clinical diagnosis of MEN-1 also should receive genetic testing for MEN-1. In addition to testing for MEN-1 and FHH, if there is clinical suspicion for hereditary hyperparathyroidism, additional genetic testing can be considered, including the following: RET (MEN-2), CDKN1B (MEN-4), GCM2 (familial isolated hyperparathyroidism type 4), and CDC73 (hyperparathyroidism-jaw tumor syndrome).¹² Some laboratories offer gene panel testing, which allows for simultaneous testing for multiple genes responsible for hereditary hyperparathyroidism.

Conclusions

In summary, clinical and biochemical parameters are not always sufficient for discriminating between pHPT and FHH. In addition, factors that influence biochemical testing (e.g., ethnicity, drugs) should be considered when interpreting test results. If a family history of hypercalcemia is known, Marx et al. proposed the diagnosis of FHH based on characteristic features within a family.¹³ Clinical red flags for FHH include asymptomatic hypercalcemia beginning in early life, relative hypocalciuria, and multiple affected family members in an autosomal dominant inheritance pattern.¹ 

In cases where the diagnosis remains uncertain or would be important for clinical decision-making, genetic testing is becoming a more frequent option. Genetic testing for FHH recently has expanded to cover not only FHH1 (CASR mutations) but also FHH2 (GNA11) and FHH3 (AP2S1) mutations. When clinical findings are suggestive of FHH, the diagnosis occurs in the context of familial hyperparathy-roidism, and when FHH testing is negative, consideration should be given to testing additional genes associated with familial hyperparathyroidism (MEN-1, RET, CDKN1B, GCM2, and CDC73). Sporadically occurring new mutations causing FHH are not uncommon. For such patients, when genetic test results are negative, it is important to obtain additional family history and calcium testing in family members. It also may be helpful to follow these patients over several years with lab tests (i.e., for calcium and PTH) and monitor them for complications like low BMD. 

At UPMC, our dedicated Endocrine Genetics Clinic combines the expertise of endocrinologists, endocrine surgeons, and clinical geneticists in a team-based approach to treating endocrine disorders with potential underlying genetic contributions. Patients and their family members can schedule multidisciplinary group appointments to address the specific complexities of genetic disorders. Our team members provide advice to patients as well as to family members. We assist with decision-making related to genetic testing, and the psychological and financial implications of such testing. In the era of genetics and personalized medicine, in which genetic contributions to disease are increasingly identified, it is more important than ever to have an integrated approach that brings genetics to the patient in the clinic. The UPMC Endocrine Genetics Clinic, which has been operating for more than a decade, is at the forefront of providing this integrated care to our patients with genetic endocrine disorders.

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