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Difficulty: MediumEndocrinology and Metabolism

During an endocrinology ward rotation, an attending physician tests the team's ability to interpret mineral metabolism panels. The team must differentiate various etiologies of abnormal calcium levels based on specific biochemical markers. Match each clinical diagnosis with the laboratory profile that best characterizes it.

  • Primary hyperparathyroidismElevated serum calcium, elevated intact PTH, and elevated 24-hour urinary calcium excretion
  • Secondary hyperparathyroidism (due to chronic kidney disease)Low to normal serum calcium, elevated intact PTH, elevated serum phosphorus, and low 1,25-dihydroxyvitamin D
  • Familial hypocalciuric hypercalcemiaMildly elevated serum calcium, inappropriately normal or elevated intact PTH, and abnormally low 24-hour urinary calcium excretion
  • Humoral hypercalcemia of malignancyElevated serum calcium, suppressed intact PTH, low serum phosphorus, and elevated PTH-related peptide

Answer

Primary hyperparathyroidism matches with elevated serum calcium, elevated intact PTH, and elevated urinary calcium. Secondary hyperparathyroidism matches with low/normal serum calcium, elevated intact PTH, elevated phosphorus, and low 1,25-dihydroxyvitamin D. Familial hypocalciuric hypercalcemia matches with mildly elevated calcium, normal/elevated intact PTH, and abnormally low urinary calcium. Humoral hypercalcemia of malignancy matches with elevated calcium, suppressed intact PTH, low phosphorus, and elevated PTH-related peptide.
Correctly matching these diagnoses requires applying the physiological feedback loops of mineral metabolism. Primary hyperparathyroidism involves autonomous PTH hypersecretion causing hypercalcemia and paradoxically high urine calcium (due to high filtered load). FHH alters the calcium set-point, resulting in hypercalcemia, unsuppressed PTH, and pathognomonic hypocalciuria. Humoral hypercalcemia of malignancy is driven by ectopic PTHrP, which causes hypercalcemia and subsequent complete suppression of endogenous intact PTH. Secondary hyperparathyroidism in chronic kidney disease is a compensatory response to hypocalcemia driven by phosphate retention and absent calcitriol.

Step-by-Step Solution

1
Evaluate the serum calcium and intact PTH levels to differentiate parathyroid-dependent from parathyroid-independent hypercalcemia.
High calcium with suppressed PTH indicates a parathyroid-independent cause (e.g., malignancy), while high calcium with normal or elevated PTH indicates a parathyroid-dependent cause (e.g., primary hyperparathyroidism, FHH).
Normal physiological feedback dictates that intact PTH should be fully suppressed in the presence of hypercalcemia. Failure to suppress implies either autonomous PTH secretion or a defect in calcium sensing.
2
Use 24-hour urinary calcium excretion to differentiate between parathyroid-dependent causes of hypercalcemia.
High urinary calcium excretion suggests primary hyperparathyroidism, whereas abnormally low urinary calcium excretion indicates familial hypocalciuric hypercalcemia (FHH).
In FHH, defective calcium-sensing receptors in the kidney lead to excessive calcium reabsorption, yielding hypocalciuria despite hypercalcemia. In primary hyperparathyroidism, the massive filtered load of calcium overwhelms reabsorption, causing hypercalciuria.
3
Identify the characteristic pattern for secondary hyperparathyroidism in the context of chronic kidney disease.
Low or normal serum calcium combined with elevated PTH, elevated phosphorus, and low calcitriol points to secondary hyperparathyroidism.
Chronic kidney disease impairs renal phosphate excretion and the synthesis of 1,25-dihydroxyvitamin D, leading to a state of hypocalcemia that chronically drives compensatory PTH hypersecretion.

Key Concept

Differential diagnosis of calcium and parathyroid hormone disorders based on characteristic laboratory profiles
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Topics

Endocrinology and Metabolism

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