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Zorluk: ZorInterpreting Laboratory Results

A 28-year-old man comes to the clinic to establish care. He feels well and has no significant past medical history. He takes no medications and does not use illicit substances. His family history is notable for a father who underwent an unsuccessful parathyroidectomy for asymptomatic hypercalcemia. Vital signs are within normal limits, and physical examination is unremarkable. Laboratory results are as follows:

Serum chemistry:
Calcium: 11.4 mg/dL
Phosphorus: 2.8 mg/dL
Creatinine: 1.0 mg/dL
Albumin: 4.0 g/dL

Endocrine panel:
Intact parathyroid hormone (PTH): 72 pg/mL (Reference: 10–65 pg/mL)
25-hydroxyvitamin D: 38 ng/mL (Reference: 20–50 ng/mL)

Urine chemistry (24-hour collection):
Calcium: 60 mg/24 hr (Reference: 100–300 mg/24 hr)
Creatinine: 1,500 mg/24 hr

Based on the interpretation of these laboratory parameters, which of the following is the most likely diagnosis?

  1. A
    Primary hyperparathyroidism
  2. Familial hypocalciuric hypercalcemiaCevap
  3. C
    Secondary hyperparathyroidism
  4. D
    Malignancy-associated hypercalcemia
  5. E
    Milk-alkali syndrome

Cevap

Familial hypocalciuric hypercalcemia
The correct answer is Familial hypocalciuric hypercalcemia (FHH). This patient presents with mild, asymptomatic hypercalcemia and an elevated intact parathyroid hormone (PTH) level, indicating a PTH-dependent etiology. The critical step in interpreting these laboratory results is calculating the calcium/creatinine clearance ratio (UCCR) to differentiate FHH from primary hyperparathyroidism. Using the provided 24-hour values, the formula is:

UCCR=UCa×PCrPCa×UCrUCCR = \frac{U_{Ca} \times P_{Cr}}{P_{Ca} \times U_{Cr}}

UCCR=60×1.011.4×1500=6017100≈0.0035UCCR = \frac{60 \times 1.0}{11.4 \times 1500} = \frac{60}{17100} \approx 0.0035

A UCCR <0.01< 0.01 is highly indicative of FHH, an autosomal dominant disorder caused by an inactivating mutation of 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. The family history of an unsuccessful parathyroidectomy further supports this, as surgery is not curative for FHH.

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1
Analyze the serum calcium and PTH relationship.
The patient has hypercalcemia (11.4 mg/dL) with an elevated PTH (72 pg/mL).
Determining whether hypercalcemia is PTH-dependent (PTH is inappropriately normal or high) or PTH-independent (PTH is appropriately suppressed) narrows the differential diagnosis. Here, it is PTH-dependent.
2
Evaluate the 24-hour urinary calcium excretion and calculate the calcium/creatinine clearance ratio (UCCR).
Urinary calcium is low (60 mg/24 hr). The UCCR is calculated as 0.0035.
In PTH-dependent hypercalcemia, calculating the UCCR differentiates primary hyperparathyroidism from familial hypocalciuric hypercalcemia (FHH). The formula is UCCR=Urine Calcium×Serum CreatinineSerum Calcium×Urine CreatinineUCCR = \frac{\text{Urine Calcium} \times \text{Serum Creatinine}}{\text{Serum Calcium} \times \text{Urine Creatinine}}.
3
Interpret the calculated UCCR in the clinical context.
A UCCR < 0.01 indicates FHH, consistent with the patient's family history of a failed parathyroidectomy.
FHH is caused by an inactivating mutation in the calcium-sensing receptor (CaSR), leading to higher calcium thresholds for PTH suppression and increased renal calcium reabsorption.

Anahtar Kavram

The differentiation of PTH-dependent hypercalcemia utilizing the calcium/creatinine clearance ratio (UCCR).
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Konular

Interpreting Laboratory Results

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