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Difficulty: HardEndocrinology and Metabolism

Match the following sets of laboratory profiles to the most likely underlying pathophysiologic diagnosis. Assume all patients have normal renal function (serum creatinine 0.7-1.0 mg/dL). Reference ranges: Serum calcium 8.4-10.2 mg/dL; Serum phosphate 2.7-4.5 mg/dL; Intact PTH 10-65 pg/mL; 24-hr urine calcium 100-300 mg.

  • Ca: 11.3 mg/dL, PO4: 2.2 mg/dL, PTH: 85 pg/mL, 24-hr Urine Ca: 380 mgPrimary hyperparathyroidism
  • Ca: 11.1 mg/dL, PO4: 2.9 mg/dL, PTH: 62 pg/mL, 24-hr Urine Ca: 45 mgFamilial hypocalciuric hypercalcemia
  • Ca: 7.4 mg/dL, PO4: 5.8 mg/dL, PTH: 215 pg/mL, 24-hr Urine Ca: 60 mgPseudohypoparathyroidism
  • Ca: 8.0 mg/dL, PO4: 2.1 mg/dL, PTH: 140 pg/mL, 24-hr Urine Ca: 55 mgVitamin D deficiency

Answer

Profile 1 matches Primary hyperparathyroidism; Profile 2 matches Familial hypocalciuric hypercalcemia; Profile 3 matches Pseudohypoparathyroidism; Profile 4 matches Vitamin D deficiency.
Each laboratory profile uniquely matches a distinct pathophysiologic state of calcium and phosphate homeostasis. Profile 1 shows autonomous PTH secretion with high urinary calcium (Primary hyperparathyroidism). Profile 2 shows an altered calcium set-point with hypocalciuria (Familial hypocalciuric hypercalcemia). Profile 3 shows target organ resistance to PTH (Pseudohypoparathyroidism). Profile 4 shows defective intestinal absorption leading to secondary hyperparathyroidism and hypophosphatemia (Vitamin D deficiency).

Step-by-Step Solution

1
Evaluate the first laboratory profile.
Identified as primary hyperparathyroidism.
The elevated calcium should normally suppress PTH. Elevated PTH in the setting of hypercalcemia, along with hypercalciuria (due to the increased filtered load of calcium exceeding the maximum reabsorption capacity), strongly indicates primary hyperparathyroidism.
2
Evaluate the second laboratory profile.
Identified as familial hypocalciuric hypercalcemia (FHH).
The presence of hypercalcemia, a high-normal PTH, and markedly low 24-hour urine calcium (< 100 mg/24hr) is the classic hallmark of FHH, which is caused by an inactivating mutation in the calcium-sensing receptor (CaSR).
3
Evaluate the third laboratory profile.
Identified as pseudohypoparathyroidism.
Normal kidneys excrete phosphate in response to PTH. Hypocalcemia and hyperphosphatemia despite markedly elevated PTH indicate end-organ resistance to PTH at the level of the kidney.
4
Evaluate the fourth laboratory profile.
Identified as vitamin D deficiency.
Lack of vitamin D leads to decreased intestinal calcium absorption. The resulting hypocalcemia triggers secondary hyperparathyroidism, which increases renal phosphate excretion, leading to hypophosphatemia alongside the hypocalcemia.

Key Concept

Differential diagnosis of calcium and phosphate disorders based on the parathyroid hormone axis.
Estimated Time:1m 30s
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Topics

Endocrinology and Metabolism

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