A 32-year-old woman comes to the clinic for a routine follow-up. She feels well and has no specific complaints. She has a history of mild depression treated with sertraline for the past two years. Her family history is notable for a father who had "high calcium levels" but never required surgery. Her blood pressure is 118/76 mm Hg, and her pulse is 72/min. Physical examination is unremarkable.
Laboratory studies show:
| Test | Result | Reference Range |
|---|---|---|
| Serum calcium | 11.2 mg/dL | 8.4-10.2 mg/dL |
| Serum albumin | 4.0 g/dL | 3.5-5.0 g/dL |
| Serum phosphorus | 2.4 mg/dL | 2.5-4.5 mg/dL |
| Intact parathyroid hormone | 68 pg/mL | 10-65 pg/mL |
| 25-hydroxyvitamin D | 32 ng/mL | 20-50 ng/mL |
| Serum creatinine | 0.8 mg/dL | 0.6-1.2 mg/dL |
Which of the following is the most appropriate next step in diagnostic evaluation?
- AParathyroid sestamibi scintigraphy
- BDual-energy x-ray absorptiometry scan
- 24-hour urinary calcium and creatinine excretionCevap
- DGenetic testing for calcium-sensing receptor mutation
- ESerum parathyroid hormone-related peptide level
Cevap
24-hour urinary calcium and creatinine excretion
The most appropriate next step in the evaluation of this patient is to measure 24-hour urinary calcium and creatinine excretion. The patient has mild hypercalcemia with an inappropriately elevated parathyroid hormone (PTH) level, indicating PTH-dependent hypercalcemia. The differential diagnosis for this presentation primarily includes primary hyperparathyroidism and familial hypocalciuric hypercalcemia (FHH). The patient's family history of asymptomatic hypercalcemia without a need for surgery strongly suggests FHH. FHH is a benign, autosomal dominant disorder caused by an inactivating mutation in the calcium-sensing receptor (CaSR) in the parathyroid glands and kidneys. This leads to a higher set-point for calcium homeostasis and increased renal calcium reabsorption. It is critical to differentiate FHH from primary hyperparathyroidism because FHH does not cause long-term target organ damage, and parathyroidectomy is ineffective and contraindicated. The definitive initial test to distinguish these two entities is the calcium-to-creatinine clearance ratio (UCCR) calculated from a 24-hour urine collection. A UCCR less than 0.01 points to FHH, whereas a ratio greater than 0.02 is typical for primary hyperparathyroidism.
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Anahtar Kavram
In patients presenting with PTH-dependent hypercalcemia, calculating the urine calcium-to-creatinine clearance ratio via a 24-hour urinary calcium and creatinine excretion test is the mandatory next step to rule out familial hypocalciuric hypercalcemia prior to diagnosing primary hyperparathyroidism.