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Difficulty: MediumEndocrinology and Metabolism

Match each clinical laboratory profile concerning calcium homeostasis to the most likely underlying pathophysiologic diagnosis.

  • Hypercalcemia, elevated parathyroid hormone (PTH), hypophosphatemia, and hypercalciuriaPrimary hyperparathyroidism
  • Hypercalcemia, normal to mildly elevated PTH, and marked hypocalciuria (urine calcium to creatinine clearance ratio < 0.01)Familial hypocalciuric hypercalcemia (FHH)
  • Hypocalcemia, inappropriately low PTH, and hyperphosphatemiaPrimary hypoparathyroidism
  • Hypocalcemia, elevated PTH, hypophosphatemia, and elevated alkaline phosphataseSevere vitamin D deficiency
  • Hypercalcemia, suppressed PTH, hypophosphatemia, and elevated PTH-related peptide (PTHrP)Humoral hypercalcemia of malignancy

Answer

The accurate diagnoses correspond to distinct biochemical signatures: Primary hyperparathyroidism with hypercalciuria, FHH with marked hypocalciuria, Primary hypoparathyroidism with low PTH and hyperphosphatemia, Severe vitamin D deficiency with secondary hyperparathyroidism and elevated alkaline phosphatase, and Malignancy with suppressed endogenous PTH.
Each laboratory pattern uniquely identifies a defect in the calcium homeostasis axis. Primary hyperparathyroidism and FHH are differentiated by urine calcium excretion. Hypoparathyroidism lacks the PTH hormone entirely, while vitamin D deficiency induces a secondary hyperparathyroidism to maintain calcium at the expense of phosphate and bone. Malignancy mimics PTH action via PTHrP, raising calcium but appropriately suppressing endogenous PTH.

Step-by-Step Solution

1
Analyze the relationship between serum calcium and PTH levels.
Hypercalcemia with unsuppressed PTH indicates a primary parathyroid disorder or CaSR defect. Hypercalcemia with suppressed PTH indicates an exogenous source or malignancy. Hypocalcemia with low PTH indicates parathyroid failure. Hypocalcemia with high PTH indicates an appropriate physiologic response to calcium loss or malabsorption.
The direction of the PTH response distinguishes primary parathyroid disorders from secondary responses to deranged calcium homeostasis.
2
Differentiate the hypercalcemic states with unsuppressed PTH by evaluating urinary calcium.
Hypercalciuria points to primary hyperparathyroidism, whereas pronounced hypocalciuria points to familial hypocalciuric hypercalcemia (FHH).
FHH involves a mutated calcium-sensing receptor that intrinsically increases renal calcium reabsorption, whereas primary hyperparathyroidism increases the total filtered load of calcium, spilling it into the urine.
3
Differentiate the hypocalcemic states based on phosphate and alkaline phosphatase.
Inappropriately low PTH with hyperphosphatemia confirms primary hypoparathyroidism. Elevated PTH with hypophosphatemia and elevated alkaline phosphatase confirms secondary hyperparathyroidism, typically due to severe vitamin D deficiency.
Vitamin D deficiency causes malabsorption of calcium, triggering compensatory PTH elevation which normalizes calcium through bone turnover but worsens phosphate wasting.

Key Concept

Calcium Homeostasis and Parathyroid Disorders
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Topics

Endocrinology and Metabolism

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