Parathyroid Hormone and Vitamin D Axis

The parathyroid hormone and vitamin D axis is the principal endocrine system regulating calcium and phosphate. Parathyroid hormone responds within minutes to changes in ionised calcium, while vitamin D, activated by the kidney, governs intestinal mineral absorption; together they act on gut, kidney, and bone to defend serum calcium.

Definition

The parathyroid hormone and vitamin D axis is the feedback system in which parathyroid hormone, secreted in response to low ionised calcium, raises calcium through actions on bone and kidney and by stimulating renal synthesis of active vitamin D (1,25-dihydroxyvitamin D), which in turn increases intestinal calcium and phosphate absorption and feeds back to suppress parathyroid hormone.

Scope

This entry describes the two central calciotropic hormones, how their secretion is regulated, how they act on target organs, and how they interact in a feedback loop. It is a reference account of the axis and its physiology; it does not provide thresholds for diagnosing deficiency or recommendations for supplementation in individuals.

Core questions

• How is parathyroid hormone secretion regulated by serum calcium?
• How is vitamin D activated, and what does the active hormone do?
• How do parathyroid hormone and vitamin D interact in a feedback loop?
• What are the consequences of vitamin D deficiency for this axis?

Key concepts

• Parathyroid hormone secretion
• Calcium-sensing receptor
• Vitamin D activation (25- and 1-hydroxylation)
• 1,25-dihydroxyvitamin D (calcitriol)
• Intestinal calcium absorption
• Secondary hyperparathyroidism
• Negative feedback regulation

Mechanisms

A fall in ionised calcium, detected by the calcium-sensing receptor on parathyroid cells, triggers parathyroid hormone secretion. Parathyroid hormone increases renal calcium reabsorption, promotes phosphate excretion, mobilises calcium from bone by increasing osteoclastic resorption (acting indirectly through osteoblast-lineage cells and the RANKL system), and stimulates the renal enzyme that converts 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D. The active vitamin D hormone then enhances intestinal absorption of calcium and phosphate and, when calcium is restored, suppresses parathyroid hormone secretion, closing the feedback loop. Vitamin D itself is obtained from skin synthesis and diet and must be hydroxylated first in the liver and then in the kidney to become active. When vitamin D is deficient, intestinal calcium absorption falls and parathyroid hormone rises compensatorily, a state of secondary hyperparathyroidism.

Clinical relevance

This axis explains disorders ranging from vitamin D deficiency and secondary hyperparathyroidism to the hypercalcaemia of primary hyperparathyroidism, and it is the target of several bone therapies. The entry provides the physiological framework behind calcium, phosphate, parathyroid hormone, and vitamin D measurements; it is descriptive reference material and not advice on testing or supplementing any individual.

Evidence & guidelines

Endocrine Society clinical practice guidance addresses the evaluation and prevention of vitamin D deficiency (Holick et al., 2011), while systematic-review evidence has examined the skeletal effects of vitamin D supplementation on bone mineral density (Reid et al., 2014). These define the clinical context of the axis, which the present entry frames physiologically rather than prescriptively.

History

The identification of vitamin D and its activation through sequential hydroxylation in liver and kidney established the hormonal nature of the vitamin, while parathyroid hormone was recognised as the rapid defender of serum calcium. The cloning of the calcium-sensing receptor in the early 1990s explained how parathyroid cells set their output to the prevailing calcium level, integrating the two hormones into a coherent feedback system.

Debates

What level of vitamin D is sufficient, and who benefits from supplementation?

Definitions of vitamin D sufficiency and the skeletal benefit of routine supplementation are contested; meta-analytic evidence has found limited effects of supplements on bone mineral density at the population level, in tension with guideline thresholds aimed at avoiding deficiency.

Key figures

• Michael Holick
• Hector DeLuca
• Edward Brown
• Mark Haussler

Seminal works

• holick-2007
• holick-2011

Frequently asked questions

What is the difference between vitamin D and its active form?

Vitamin D from skin or diet is biologically inactive; the liver converts it to 25-hydroxyvitamin D (the form measured to assess status), and the kidney converts that to 1,25-dihydroxyvitamin D, the active hormone that drives intestinal calcium absorption.

Why does parathyroid hormone rise when vitamin D is low?

Low vitamin D reduces intestinal calcium absorption, which tends to lower serum calcium; the parathyroid glands respond by secreting more parathyroid hormone, a compensatory state called secondary hyperparathyroidism.