A 32-year-old woman is evaluated in the clinic following a routine wellness examination that revealed an elevated serum calcium level. She has no complaints and reports feeling well. She has no history of nephrolithiasis, bone fractures, or abdominal pain. She takes no medications or over-the-counter supplements. Vital signs are normal. Physical examination is unremarkable.
Laboratory results are as follows:
| Laboratory Test | Result | Reference Range |
|---|---|---|
| Calcium, serum | 11.1 mg/dL | 8.4–10.2 mg/dL |
| Albumin, serum | 4.0 g/dL | 3.5–5.0 g/dL |
| Intact parathyroid hormone | 72 pg/mL | 10–65 pg/mL |
| 25-hydroxyvitamin D | 34 ng/mL | 20–50 ng/mL |
| Creatinine, serum | 0.8 mg/dL | 0.6–1.2 mg/dL |
| Calcium, 24-hour urine | 60 mg/day | 100–300 mg/day |
Which of the following is the most likely diagnosis?
- APrimary hyperparathyroidism
- BHumoral hypercalcemia of malignancy
- Familial hypocalciuric hypercalcemiaAnswer
- DSecondary hyperparathyroidism
- EMilk-alkali syndrome
Answer
Familial hypocalciuric hypercalcemia
The patient's laboratory results show mild hypercalcemia, mildly elevated intact parathyroid hormone (PTH), and significantly low 24-hour urine calcium excretion (<100 mg/day). This pattern is characteristic of familial hypocalciuric hypercalcemia (FHH). FHH is an autosomal dominant condition caused by an inactivating mutation in the calcium-sensing receptor (CaSR) in the parathyroid glands and kidneys. The defective receptor requires higher serum calcium levels to suppress PTH release, leading to inappropriately normal or mildly elevated PTH. In the kidneys, the defective receptor leads to increased tubular reabsorption of calcium, causing hypocalciuria. Differentiating FHH from primary hyperparathyroidism is critical because FHH is a benign condition that does not require parathyroidectomy.
Step-by-Step Solution
Key Concept
Familial hypocalciuric hypercalcemia (FHH) presents with mild hypercalcemia, normal or slightly elevated parathyroid hormone (PTH), and significant hypocalciuria due to an inactivating mutation in the calcium-sensing receptor.