A 29-year-old woman is evaluated in the clinic as a new patient. She has no complaints and… — USMLE Step 2 CK

A 29-year-old woman is evaluated in the clinic as a new patient. She has no complaints and reports feeling well. She exercises regularly, follows a balanced diet, and takes no medications or supplements. Her father has a history of asymptomatic hypercalcemia that has never required medical treatment. Vital signs are within normal limits. 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	$4.1$ g/dL	$3.5-5.0$ g/dL
Serum phosphorus	$2.3$ mg/dL	$2.5-4.5$ mg/dL
Serum creatinine	$0.8$ mg/dL	$0.6-1.1$ mg/dL
Intact parathyroid hormone	$68$ pg/mL	$10-65$ pg/mL

A 24-hour urine collection yields:

Test	Result	Reference Range
Urine calcium	$65$ mg/24 hr	$100-300$ mg/24 hr
Urine creatinine	$1000$ mg/24 hr	$800-1500$ mg/24 hr

Which of the following is the most appropriate next step in management?

Reassurance and clinical observationAnswer
B
Referral for parathyroidectomy
C
Sestamibi parathyroid scintigraphy
D
Dual-energy x-ray absorptiometry
E
Measurement of serum 1,25-dihydroxyvitamin D

Answer

Reassurance and clinical observation

The correct approach is reassurance and clinical observation. This patient presents with asymptomatic hypercalcemia and an intact parathyroid hormone (PTH) level that is mildly elevated, indicating a PTH-dependent etiology. The critical distinguishing factor is the very low 24-hour urine calcium (

65

mg/24 hr) in the setting of hypercalcemia.

To formally differentiate between the two most likely causes—primary hyperparathyroidism and familial hypocalciuric hypercalcemia (FHH)—the calcium-creatinine clearance ratio (CCCR) must be calculated:

CCCR = \frac{\text{Urine Ca} \times \text{Serum Cr}}{\text{Serum Ca} \times \text{Urine Cr}}

CCCR = \frac{65 \times 0.8}{11.2 \times 1000} = \frac{52}{11200} \approx 0.0046

A CCCR of

< 0.01

is highly sensitive and specific for FHH, whereas primary hyperparathyroidism typically yields a CCCR

> 0.02

. Furthermore, the patient's family history of asymptomatic, untreated hypercalcemia strongly supports this autosomal dominant condition. FHH is caused by an inactivating mutation of the calcium-sensing receptor (CaSR) in the parathyroid glands and kidneys, requiring higher calcium levels to suppress PTH and increasing renal calcium reabsorption. It is a benign condition that does not cause progressive bone loss or renal stones; therefore, medical or surgical interventions are not indicated, and the best step is reassurance.

Step-by-Step Solution

Analyze the serum laboratory findings to determine the category of hypercalcemia.

The patient has hypercalcemia (

11.2

mg/dL) with an inappropriately high-normal/mildly elevated intact PTH (

68

pg/mL).

Elevated or high-normal PTH in the setting of hypercalcemia indicates a PTH-dependent process, narrowing the differential diagnosis primarily to primary hyperparathyroidism and familial hypocalciuric hypercalcemia (FHH).

Calculate the calcium-creatinine clearance ratio (CCCR) using the 24-hour urine data to differentiate between primary hyperparathyroidism and FHH.

The CCCR is calculated as:

CCCR = \frac{\text{Urine Ca} \times \text{Serum Cr}}{\text{Serum Ca} \times \text{Urine Cr}}

. Here,

(65 \times 0.8) / (11.2 \times 1000) = 52 / 11200 \approx 0.0046

A CCCR

< 0.01

is highly suggestive of FHH, whereas primary hyperparathyroidism typically presents with a CCCR

> 0.02

Determine the appropriate management for the diagnosed condition.

The patient is diagnosed with FHH. The most appropriate next step is reassurance and clinical observation.

FHH is a benign, asymptomatic condition caused by an inactivating mutation in the calcium-sensing receptor (CaSR). It does not lead to end-organ damage, and parathyroidectomy is neither curative nor indicated.

Key Concept

Familial hypocalciuric hypercalcemia (FHH) presents with PTH-dependent hypercalcemia but is distinguished from primary hyperparathyroidism by a very low 24-hour urine calcium and a calcium-creatinine clearance ratio (CCCR) of

< 0.01

. It is a benign condition that requires only reassurance and observation.

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