A 42-year-old man comes to the clinic for a follow-up evaluation. Two weeks ago, he was no… — USMLE Step 3

A 42-year-old man comes to the clinic for a follow-up evaluation. Two weeks ago, he was noted to have an elevated serum calcium level on a routine comprehensive metabolic panel obtained for a life insurance policy. He feels well and has no fatigue, abdominal pain, bone pain, or changes in bowel habits. His only medical history is mild hypertension, for which he has been taking amlodipine for three years. He does not take any over-the-counter supplements. He is adopted and does not know his family medical history. Blood pressure is 128/82 mm Hg, and pulse is 72/min. Physical examination is unremarkable.

Laboratory results are as follows:

Test	Result	Reference Range
Serum
Calcium	11.0 mg/dL	8.4-10.2 mg/dL
Albumin	4.0 g/dL	3.5-5.0 g/dL
Phosphorus	2.3 mg/dL	2.5-4.5 mg/dL
Creatinine	1.0 mg/dL	0.7-1.3 mg/dL
Intact parathyroid hormone	68 pg/mL	10-65 pg/mL
25-hydroxyvitamin D	35 ng/mL	20-50 ng/mL
Spot Urine
Calcium	5 mg/dL	-
Creatinine	100 mg/dL	-

Based on the laboratory results, which of the following is the most likely underlying mechanism for this patient's findings?

Inactivating mutation in the calcium-sensing receptorAnswer
B
Monoclonal proliferation of parathyroid chief cells
C
Ectopic secretion of parathyroid hormone-related peptide
D
Autonomous production of 1,25-dihydroxyvitamin D
E
Decreased renal calcium excretion secondary to distal tubule transporter blockade

Answer

The most likely underlying mechanism is an inactivating mutation in the calcium-sensing receptor, resulting in Familial Hypocalciuric Hypercalcemia.

The patient has mild hypercalcemia with an inappropriately elevated intact parathyroid hormone (PTH) level, indicating PTH-dependent hypercalcemia. In an asymptomatic patient, the two primary considerations are primary hyperparathyroidism and familial hypocalciuric hypercalcemia (FHH). To differentiate these, the calcium-to-creatinine clearance ratio (CCCR) must be calculated using the formula:

CCCR = \frac{Urine_{Ca} \times Serum_{Cr}}{Serum_{Ca} \times Urine_{Cr}}

. For this patient,

CCCR = \frac{5 \times 1.0}{11.0 \times 100} = 0.0045

. A ratio less than 0.01 (1%) is highly suggestive of FHH, an autosomal dominant disorder caused by an inactivating mutation in the calcium-sensing receptor (CaSR) located in the parathyroid glands and renal tubules. This mutation alters the set-point for calcium homeostasis, requiring higher serum calcium levels to suppress PTH secretion, and simultaneously increases renal calcium reabsorption, leading to marked hypocalciuria.

Step-by-Step Solution

Evaluate the initial serum calcium and parathyroid hormone (PTH) levels.

The patient has hypercalcemia (11.0 mg/dL) with an inappropriately high-normal to mildly elevated PTH (68 pg/mL).

This establishes the presence of PTH-dependent hypercalcemia, immediately narrowing the primary differential diagnosis to primary hyperparathyroidism and familial hypocalciuric hypercalcemia (FHH).

Calculate the calcium-to-creatinine clearance ratio (CCCR) to differentiate the remaining etiologies.

CCCR = (Urine Calcium × Serum Creatinine) / (Serum Calcium × Urine Creatinine) = (5 × 1.0) / (11.0 × 100) = 5 / 1100 = 0.0045.

The CCCR is an essential diagnostic index for PTH-dependent hypercalcemia. A CCCR < 0.01 strongly suggests FHH, while a CCCR > 0.02 suggests primary hyperparathyroidism.

Correlate the calculated CCCR with the underlying pathophysiology.

A CCCR of 0.0045 (< 0.01) confirms the diagnosis of FHH, which is caused by an inactivating mutation in the calcium-sensing receptor (CaSR).

This mutation requires higher serum calcium levels to suppress PTH secretion and causes increased renal calcium reabsorption, directly resulting in the observed laboratory pattern.

Key Concept

Differentiating familial hypocalciuric hypercalcemia from primary hyperparathyroidism using the calculated calcium-to-creatinine clearance ratio.

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