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Measurement of parathyroid hormone
D B Endres1, R Villanueva, C F Sharp
1School of Medicine, University of Southern California, Los Angeles.
Endocrinology and Metabolism Clinics of North America
|September 1, 1989
Summary
Understanding parathyroid hormone (PTH) requires knowledge of its secretion and metabolism. Assays measuring intact PTH offer superior specificity for diagnosing hypercalcemia compared to mid-region assays.
Area of Science:
- Endocrinology
- Clinical Chemistry
- Assay Development
Background:
- Circulating parathyroid hormone (PTH) exists as intact hormone and biologically inactive carboxyl fragments.
- Inactive fragments, produced by secretion and peripheral metabolism, constitute the majority of serum immunoreactivity due to longer half-lives.
- Immunoassays for PTH vary in their ability to measure intact hormone versus inactive fragments.
Purpose of the Study:
- To analyze the heterogeneity of circulating immunoreactive PTH and its implications for immunoassay interpretation.
- To compare the clinical utility of different PTH assay methodologies in diagnosing hypercalcemia, particularly primary hyperparathyroidism.
Main Methods:
- Review of PTH secretion, metabolism, and fragment characteristics.
- Comparison of N-terminal, intact, mid-region, and polyvalent PTH immunoassays.
- Evaluation of assay sensitivity and specificity in differentiating causes of hypercalcemia.
Main Results:
- Mid-region assays show high sensitivity for primary hyperparathyroidism but can yield elevated levels in nonparathyroid hypercalcemia.
- Intact PTH assays demonstrate improved sensitivity over conventional methods and superior specificity, correctly identifying suppressed PTH in malignancy-associated hypercalcemia.
- Both sensitive mid-region and intact PTH assays are valuable for hypercalcemia differential diagnosis.
Conclusions:
- The choice of PTH assay significantly impacts diagnostic accuracy due to differences in measuring intact hormone versus inactive fragments.
- Intact PTH assays offer better specificity for distinguishing primary hyperparathyroidism from other causes of hypercalcemia, especially malignancy.
- Understanding PTH heterogeneity is crucial for accurate interpretation of immunoassay results in clinical practice.