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Difficulty: MediumEndocrinology and Metabolism

Match each of the following serum laboratory profiles with the most likely underlying endocrinologic diagnosis.

  • Elevated serum calcium, suppressed parathyroid hormone (PTH), and elevated parathyroid hormone-related peptide (PTHrP).Humoral hypercalcemia of malignancy
  • Elevated serum calcium, inappropriately normal or elevated parathyroid hormone (PTH), and high 24-hour urinary calcium excretion.Primary hyperparathyroidism
  • Elevated serum calcium, normal to mildly elevated parathyroid hormone (PTH), and notably low 24-hour urinary calcium excretion.Familial hypocalciuric hypercalcemia
  • Decreased serum calcium, elevated parathyroid hormone (PTH), decreased serum phosphate, and low 25-hydroxyvitamin D.Severe nutritional vitamin D deficiency
  • Decreased serum calcium, inappropriately low parathyroid hormone (PTH), and elevated serum phosphate.Primary hypoparathyroidism

Answer

The laboratory profiles correctly map to Humoral hypercalcemia of malignancy (elevated calcium, suppressed PTH, elevated PTHrP), Primary hyperparathyroidism (elevated calcium, high PTH, high urinary calcium), Familial hypocalciuric hypercalcemia (elevated calcium, normal/high PTH, low urinary calcium), Severe nutritional vitamin D deficiency (low calcium, high PTH, low vitamin D), and Primary hypoparathyroidism (low calcium, low PTH, high phosphate).
The correct matches rely on the physiological feedback loops governing mineral metabolism. Hypercalcemia should normally suppress PTH; when it does, one must look for external drivers like PTHrP from malignancy. When hypercalcemia fails to suppress PTH, the defect lies in the parathyroid or the calcium sensors (differentiated by urine calcium). Conversely, hypocalcemia should stimulate PTH; if PTH is high, it is a secondary response (like in Vitamin D deficiency), but if PTH is low, the glands themselves have failed.

Step-by-Step Solution

1
Evaluate the serum calcium and parathyroid hormone (PTH) levels to determine if their relationship follows physiological feedback loops.
Identifies whether the primary defect is located in the parathyroid gland itself or is driven by an external factor.
PTH should normally be suppressed by hypercalcemia and stimulated by hypocalcemia. Deviations from this indicate autonomous parathyroid pathology or altered receptor sensing.
2
For hypercalcemic states with suppressed PTH, assess for non-parathyroid sources of hypercalcemia.
Points to conditions like malignancy (elevated PTHrP) or exogenous toxicity.
Intact feedback mechanisms properly suppress parathyroid gland activity when calcium is elevated by an external source.
3
For hypercalcemic states with inappropriately normal or elevated PTH, evaluate 24-hour urinary calcium excretion.
Differentiates primary hyperparathyroidism from familial hypocalciuric hypercalcemia (FHH).
FHH is characterized by defective calcium sensing, leading to anomalous renal calcium retention despite hypercalcemia, whereas primary hyperparathyroidism features hypercalciuria due to the massive filtered calcium load.

Key Concept

Diagnostic interpretation of calcium, phosphorus, and parathyroid hormone laboratory profiles to differentiate parathyroid disorders from other causes of disrupted mineral metabolism.
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Topics

Endocrinology and Metabolism

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