Parathyroid Hormone (PTH) and Calcium Regulation

Parathyroid hormone is the dominant minute-to-minute regulator of blood calcium. Secreted by the parathyroid glands in response to a fall in ionized calcium sensed through the calcium-sensing receptor, PTH raises calcium by mobilising bone mineral, increasing renal calcium reabsorption, and stimulating production of active vitamin D, which in turn enhances intestinal calcium absorption. Clinical laboratories measure intact PTH and interpret it together with calcium.

Definition

Parathyroid hormone is the peptide hormone of the parathyroid glands whose secretion is governed by ionized calcium through the calcium-sensing receptor and which raises serum calcium by acting on bone and kidney and by stimulating renal synthesis of active vitamin D.

Scope

This topic covers the synthesis and secretion of PTH, its calcium-sensing feedback control, its actions on bone, kidney, and (indirectly) gut, and how PTH measurement is interpreted alongside calcium. It is a physiology-and-measurement reference; it does not provide diagnostic or treatment protocols.

Core questions

• How does the calcium-sensing receptor couple ionized calcium to PTH secretion?
• Through what actions on bone, kidney, and gut does PTH raise serum calcium?
• Why is PTH interpreted together with the concurrent calcium value?
• What does the assay's measurement of 'intact' PTH represent?

Key concepts

• Calcium-sensing receptor feedback
• Intact PTH assay
• PTH actions on bone (calcium mobilisation)
• Renal calcium reabsorption and phosphaturia
• Stimulation of 1-alpha-hydroxylase (active vitamin D)
• Paired calcium-PTH interpretation
• Primary versus secondary hyperparathyroidism (concept)

Mechanisms

Parathyroid chief cells continuously monitor ionized calcium through the calcium-sensing receptor; a fall in calcium relieves receptor-mediated suppression and increases PTH secretion. PTH then raises calcium through three routes: it promotes osteoclast-mediated mobilisation of calcium and phosphate from bone, increases calcium reabsorption while reducing phosphate reabsorption (phosphaturia) in the kidney, and stimulates renal 1-alpha-hydroxylase to convert 25-hydroxyvitamin D into the active 1,25-dihydroxy form, which raises intestinal calcium absorption. As calcium rises, calcium-sensing receptor activation suppresses further secretion, closing the loop. Because PTH and calcium move reciprocally in health, a PTH result is interpreted in light of the simultaneous calcium concentration.

Clinical relevance

The reciprocal relationship between calcium and PTH is why these analytes are read as a pair, and understanding PTH's actions explains the biochemical patterns appraised in metabolic bone and mineral disorders—part of laboratory medicine literacy. This entry describes the physiology and measurement of PTH and is not a basis for individual diagnosis or treatment.

Epidemiology

PTH measurement is widely used in evaluating disorders of calcium and bone. Primary hyperparathyroidism is a common endocrine condition, frequently detected through routine calcium testing, and secondary changes in PTH are characteristic of chronic kidney disease and vitamin D deficiency, as discussed in the supporting reviews and guideline.

History

Parathyroid hormone was isolated in the early twentieth century and its calcium-regulating role progressively defined; the cloning of the calcium-sensing receptor in 1993 explained how parathyroid cells set secretion to ionized calcium. Successive international workshops, including the fourth summarised by Bilezikian and colleagues (2014), have refined how PTH and calcium findings in primary hyperparathyroidism are understood.

Debates

How should PTH assays be standardised across generations and platforms?

Different 'intact' PTH immunoassays detect somewhat different mixtures of the hormone and its fragments, so values are not always interchangeable, which complicates comparison and continues to motivate calls for assay harmonisation.

Key figures

• William D. Fraser
• John P. Bilezikian
• Munro Peacock

Seminal works

• peacock-2010
• fraser-2009
• bilezikian-2014

Frequently asked questions

Why are calcium and PTH measured and interpreted together?

PTH secretion is driven by ionized calcium through a feedback loop, so the two normally move in opposite directions; a PTH value is meaningful only when read against the calcium concentration measured at the same time.

How does PTH raise blood calcium?

PTH mobilises calcium from bone, increases renal calcium reabsorption while promoting phosphate excretion, and stimulates the kidney to make active vitamin D, which increases calcium absorption from the intestine.