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Difficulty: HardEndocrinology and Metabolism

Review the four distinct patient laboratory profiles concerning calcium and bone metabolism. Reference ranges: Calcium 8.4-10.2 mg/dL; Phosphorus 2.5-4.5 mg/dL; Intact PTH 15-65 pg/mL; 25-hydroxyvitamin D 20-50 ng/mL; 1,25-dihydroxyvitamin D 18-72 pg/mL. Assign each laboratory profile to the most accurate underlying pathophysiologic etiology.

  • Profile 1: Calcium 11.2 mg/dL, phosphorus 2.4 mg/dL, intact PTH 10 pg/mL, 1,25-dihydroxyvitamin D 12 pg/mLHumoral hypercalcemia of malignancy
  • Profile 2: Calcium 11.6 mg/dL, phosphorus 2.2 mg/dL, intact PTH 125 pg/mL, urine calcium-to-creatinine clearance ratio (UCCR) 0.035Primary hyperparathyroidism
  • Profile 3: Calcium 11.1 mg/dL, phosphorus 3.2 mg/dL, intact PTH 58 pg/mL, urine calcium-to-creatinine clearance ratio (UCCR) 0.006Familial hypocalciuric hypercalcemia
  • Profile 4: Calcium 8.1 mg/dL, phosphorus 2.1 mg/dL, intact PTH 155 pg/mL, 25-hydroxyvitamin D 9 ng/mLSevere vitamin D deficiency

Answer

Profile 1 correlates with Humoral hypercalcemia of malignancy; Profile 2 correlates with Primary hyperparathyroidism; Profile 3 correlates with Familial hypocalciuric hypercalcemia; Profile 4 correlates with Severe vitamin D deficiency.
The calcium-PTH axis must be interpreted stepwise. High calcium with suppressed PTH implies a non-parathyroid source (e.g., PTHrP in malignancy). High calcium with normal/elevated PTH indicates a parathyroid-dependent source, requiring UCCR to distinguish primary hyperparathyroidism from FHH. Low calcium with low phosphorus implies vitamin D deficiency, as PTH is elevated but cannot compensate without sufficient vitamin D substrate.

Step-by-Step Solution

1
Evaluate Profile 1's PTH level in the context of hypercalcemia.
PTH is appropriately suppressed (10 pg/mL).
This rules out PTH-dependent causes. The low 1,25-dihydroxyvitamin D further differentiates PTHrP-mediated hypercalcemia from extrarenal calcitriol production (e.g., sarcoidosis).
2
Analyze Profile 2 and Profile 3, which both feature hypercalcemia with unsuppressed PTH.
Profile 2 has a UCCR > 0.02, while Profile 3 has a UCCR < 0.01.
UCCR is essential to differentiate primary hyperparathyroidism (high relative urine calcium clearance) from FHH (impaired renal calcium sensing leading to hypocalciuria).
3
Assess Profile 4's calcium and phosphorus levels.
Both are low, accompanied by an elevated PTH and low 25-hydroxyvitamin D.
Low vitamin D leads to decreased intestinal calcium and phosphorus absorption, triggering a secondary rise in PTH, which further lowers phosphorus via phosphaturia.

Key Concept

Differentiation of hypercalcemic and hypocalcemic disorders using intact PTH, phosphorus, 1,25-dihydroxyvitamin D, and urine calcium-to-creatinine clearance ratio.
Estimated Time:1m 30s
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Topics

Endocrinology and Metabolism

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