Decoding Parathyroid Hormone: What the Latest Tests Reveal About Hyperparathyroidism
"A deeper dive into third-generation PTH assays and their role in diagnosing primary hyperparathyroidism, especially in diverse populations."
Parathyroid hormone (PTH) is the key regulator of calcium levels in our blood. Produced by the parathyroid glands, this 84-amino-acid peptide ensures our nerves, muscles, and bones function correctly. When these glands go into overdrive, they release too much PTH, leading to a condition called hyperparathyroidism.
Diagnosing hyperparathyroidism isn't always straightforward. Over the years, PTH testing has evolved, from the first-generation assays to the more sophisticated second- and third-generation methods. Each generation has its own way of measuring PTH, leading to different results and interpretations.
This article will discuss the advancements in PTH testing, focusing on the third-generation assays and how they compare to the older methods. Drawing on a study conducted in a Brazilian population, we'll explore the nuances of these tests and their implications for diagnosing primary hyperparathyroidism.
Early milestones in understanding hyperparathyroidism
Parathyroid hormone was discovered in 1925, the same year Mandl performed the first parathyroidectomy for primary hyperparathyroidism (PHPT). In Vienna, F. Mandl (1925) and E. Gold (1927) described patients with osteitis fibrosa cystica, hypercalcemia, and hypercalciuria; Gold had termed the condition primary hyperparathyroidism in 1928. In 1934, Albright described multigland primary parathyroid hyperplasia in 3 of the first 17 PHPT operations at Massachusetts General Hospital.
PTH Testing: How Third-Generation Assays Improve Diagnosis
The earliest PTH tests, known as first-generation assays, had a significant limitation: they couldn't distinguish between the complete PTH molecule (1-84) and its fragments. This meant that individuals with kidney issues, who often have high levels of these fragments, could receive inaccurate PTH readings.
- First-Generation Assays: Couldn't differentiate between PTH and its fragments, leading to inaccuracies.
- Second-Generation Assays: Aimed to measure intact PTH (1-84) but still detected some fragments.
- Third-Generation Assays: Designed to exclusively measure the biologically active PTH (1-84) form.
PTH assays capture different hormone fragments
Second-generation intact PTH assays detect both PTH 1-84 and the 7-84 fragment, whereas third-generation whole PTH assays do not recognize PTH 7-84. A study comparing the Roche intact PTH and Roche whole PTH (1-84) methods examines how the differences relate to eGFR. The recent international guideline on evaluating and managing primary hyperparathyroidism recommends using either second- or third-generation PTH assays. Separately, a big-data study reported that PTH measured by a second-generation immunoassay rose with age in normocalcemic people without renal dysfunction when vitamin D was above 20 ng/mL.
Assay limitations and the complexity of kidney failure
First-generation PTH assays had a major limitation: interference from PTH carboxyl fragments, especially in people with impaired renal function. Kidney failure can also make hyperparathyroidism difficult to interpret because it involves interacting changes in vitamin D, calcium, phosphate, and PTH; Merck reports that it causes low blood calcium, high phosphate, and reduced active vitamin D needed for intestinal calcium absorption. Separately, a 2025 Lancet commentary urged the Sixth International Workshop on primary hyperparathyroidism to incorporate emerging evidence and the perspectives of people with lived experience into future guidelines.
The Future of PTH Testing
As testing methodologies advance, third-generation PTH assays offer promise of more precise assessment, ultimately guiding more effective and personalized treatment strategies. While the findings suggest these tests may not always be superior, their continued refinement and integration with clinical assessments remain vital for optimizing patient care in hyperparathyroidism and related conditions.
Screening has changed how PHPT is recognized
Primary hyperparathyroidism is characterized by excess parathyroid hormone and disrupted calcium metabolism leading to hypercalcemia. It was previously recognized as a symptomatic disease with severe skeletal, renal, and neuromuscular manifestations. The article reports that routine calcium screening has shifted the clinical profile of the condition.
Cases illustrate varied routes to diagnosis
A 2004 case study describes PHPT as a hypercalcemic condition caused by excess PTH from one or more parathyroid glands, usually due to a benign adenoma and rarely to carcinoma. It reports that the condition is often discovered through routine serum chemistry showing hypercalcemia, hypophosphatemia, and elevated PTH. Another report describes PHPT caused by an ectopic parathyroid adenoma in the posterior superior mediastinum, with the patient’s PTH, calcium, and vitamin D followed for one year.