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Difficulty: EasyEndocrinology and Metabolism

Match the following laboratory profiles regarding serum calcium and intact parathyroid hormone (PTH) to their most likely underlying diagnosis.

  • Elevated serum calcium and inappropriately elevated intact PTHPrimary hyperparathyroidism
  • Elevated serum calcium and suppressed intact PTHHypercalcemia of malignancy
  • Decreased serum calcium and elevated intact PTHSecondary hyperparathyroidism
  • Decreased serum calcium and low intact PTHPrimary hypoparathyroidism

Answer

Elevated calcium and elevated PTH matches with Primary hyperparathyroidism. Elevated calcium and suppressed PTH matches with Hypercalcemia of malignancy. Decreased calcium and elevated PTH matches with Secondary hyperparathyroidism. Decreased calcium and low PTH matches with Primary hypoparathyroidism.
Matching laboratory profiles of calcium and PTH directly tests the understanding of parathyroid gland function and its feedback loop. When the parathyroid autonomously produces excess PTH, both calcium and PTH are high (primary hyperparathyroidism). When calcium is high due to other causes like malignancy, normal feedback suppresses PTH. When calcium is low, a healthy parathyroid gland increases PTH production (secondary hyperparathyroidism). If the gland is destroyed or removed, both calcium and PTH will be low (hypoparathyroidism).

Step-by-Step Solution

1
Analyze the relationship between serum calcium and intact parathyroid hormone (PTH) in each laboratory profile.
Identify whether the PTH level is an appropriate physiological response to the calcium level or if it is the primary driver of the abnormality.
Calcium and PTH operate on a tight negative feedback loop; understanding this relationship is the key to diagnosing the underlying endocrine disorder.
2
Evaluate the profiles presenting with hypercalcemia.
High calcium paired with high PTH indicates primary hyperparathyroidism. High calcium paired with suppressed PTH indicates a non-parathyroid cause, most classically hypercalcemia of malignancy.
Autonomous PTH secretion drives calcium up in primary hyperparathyroidism, whereas in malignancy, the high calcium successfully suppresses healthy parathyroid tissue.
3
Evaluate the profiles presenting with hypocalcemia.
Low calcium paired with high PTH indicates a compensatory response known as secondary hyperparathyroidism. Low calcium paired with low PTH indicates failure of the parathyroid glands (primary hypoparathyroidism).
A healthy parathyroid gland will predictably respond to hypocalcemia by increasing PTH secretion. The absence of this response confirms gland failure.

Key Concept

The relationship between serum calcium and intact parathyroid hormone is determined by negative feedback; deviations from this relationship differentiate primary, secondary, and non-parathyroid disorders of calcium metabolism.
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Topics

Endocrinology and Metabolism

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