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Difficulty: MediumInterpreting Laboratory Results

Match each of the following disorders of calcium and bone metabolism to its most characteristic pattern of laboratory findings.

  • Primary hyperparathyroidismElevated serum calcium, elevated intact PTH, decreased serum phosphate, elevated 24-hour urine calcium
  • Secondary hyperparathyroidism (due to chronic kidney disease)Decreased or low-normal serum calcium, elevated intact PTH, elevated serum phosphate, decreased 1,25-dihydroxyvitamin D
  • Familial hypocalciuric hypercalcemia (FHH)Elevated serum calcium, normal or mildly elevated intact PTH, normal serum phosphate, severely decreased 24-hour urine calcium
  • Humoral hypercalcemia of malignancyElevated serum calcium, suppressed intact PTH, decreased serum phosphate, elevated PTH-related peptide (PTHrP)

Answer

Primary hyperparathyroidism matches with elevated calcium, elevated PTH, and high urine calcium. Secondary hyperparathyroidism matches with low calcium, elevated PTH, and high phosphate. FHH matches with elevated calcium, normal PTH, and low urine calcium. Humoral hypercalcemia matches with elevated calcium, suppressed PTH, and elevated PTHrP.
Each condition features a distinct disruption of the calcium-PTH axis. Primary hyperparathyroidism is driven by autonomous PTH secretion; FHH by a mutated calcium sensor altering the physiological set-point; malignancy by ectopic PTHrP mimicking PTH activity; and chronic kidney disease by phosphate retention driving a compensatory PTH increase.

Step-by-Step Solution

1
Analyze the serum calcium and intact PTH directionality to distinguish between primary parathyroid disorders and compensatory or independent mechanisms.
High calcium with unsuppressed (high or normal) PTH indicates a primary parathyroid issue or FHH. High calcium with low PTH indicates a non-parathyroid source, such as malignancy.
PTH is the primary physiological regulator of serum calcium; understanding their feedback loop relationship defines the broad diagnostic category.
2
Evaluate the 24-hour urine calcium to differentiate primary hyperparathyroidism from familial hypocalciuric hypercalcemia (FHH).
High urine calcium points to primary hyperparathyroidism, while remarkably low urine calcium confirms FHH.
Both present with high serum calcium and unsuppressed PTH, but their renal handling of calcium is fundamentally opposed due to the CaSR mutation in FHH.
3
Check phosphate and vitamin D levels in cases of low calcium and high PTH.
High phosphate and low 1,25-dihydroxyvitamin D point to chronic kidney disease as the underlying cause.
This confirms secondary hyperparathyroidism driven by renal failure rather than a pure vitamin D deficiency, which would typically feature low phosphate.

Key Concept

Interpretation of the calcium-parathyroid hormone axis and associated renal/bone laboratory markers.
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Topics

Interpreting Laboratory Results

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