25-Hydroxyvitamin D and 1,25-Dihydroxyvitamin D

Vitamin D is converted in the body through two hydroxylation steps. The liver makes 25-hydroxyvitamin D (calcidiol), the main circulating form and the analyte used to assess vitamin D status because of its abundance and long half-life. The kidney then makes 1,25-dihydroxyvitamin D (calcitriol), the active hormone that drives intestinal calcium and phosphate absorption. The two metabolites answer different questions, and laboratories choose between them accordingly.

Definition

25-Hydroxyvitamin D (calcidiol) is the abundant, long-lived hepatic metabolite used to assess vitamin D status, while 1,25-dihydroxyvitamin D (calcitriol) is the short-lived, renally produced active hormone that mediates vitamin D's effects on calcium and phosphate absorption.

Scope

This topic covers the two-step activation of vitamin D, the distinct meaning of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D as analytes, their hormonal regulation, and why status assessment relies on the 25-hydroxy form. It is a measurement-and-physiology reference and does not provide diagnostic thresholds, dosing, or treatment guidance.

Core questions

• What are the two hydroxylation steps that activate vitamin D, and where do they occur?
• Why is 25-hydroxyvitamin D, not 1,25-dihydroxyvitamin D, used to assess vitamin D status?
• How do PTH, phosphate, and FGF23 regulate renal production of the active metabolite?
• What pre-analytical and assay-standardisation issues affect 25-hydroxyvitamin D measurement?

Key concepts

• Hepatic 25-hydroxylation (calcidiol)
• Renal 1-alpha-hydroxylation (calcitriol)
• 25-hydroxyvitamin D as the status marker
• Vitamin D receptor signalling
• Regulation by PTH, phosphate, and FGF23
• Assay standardisation (VDSP) and pre-analytical factors
• Vitamin D binding protein

Mechanisms

Vitamin D from skin synthesis or diet is first hydroxylated in the liver to 25-hydroxyvitamin D, which circulates bound to vitamin D binding protein and, because of its abundance and long half-life, reflects overall vitamin D supply. The kidney then converts it via 1-alpha-hydroxylase to 1,25-dihydroxyvitamin D, the active hormone that binds the vitamin D receptor to increase intestinal absorption of calcium and phosphate. Renal activation is up-regulated by parathyroid hormone and low phosphate and suppressed by FGF23 and high phosphate, integrating vitamin D activation with the calcium-phosphate-PTH network. Because the active metabolite is tightly regulated and short-lived, it does not track supply; status is therefore judged from 25-hydroxyvitamin D, whose measurement is sensitive to assay calibration.

Clinical relevance

Distinguishing the storage form from the active hormone explains why status is assessed with 25-hydroxyvitamin D while the active metabolite is reserved for specific questions—an important point of laboratory medicine literacy. Recognising assay-standardisation effects also explains why vitamin D values can differ between methods. This entry describes the biochemistry and measurement of vitamin D metabolites and is not a basis for individual diagnosis, dosing, or treatment.

Epidemiology

25-Hydroxyvitamin D is among the most commonly requested endocrine tests, reflecting widespread interest in vitamin D status across populations. Comparison of results between studies and laboratories has historically been hampered by assay variability, motivating standardisation efforts referenced in the supporting literature.

History

The two-step hepatic-then-renal activation of vitamin D and the role of 1,25-dihydroxyvitamin D as a hormone were elucidated through mid-twentieth-century biochemistry. Subsequent clinical practice guidelines, such as the Endocrine Society statement led by Holick (2011), and broad reviews of vitamin D actions, such as Bouillon and colleagues (2019), consolidated how the metabolites are measured and understood, while standardisation programmes addressed assay comparability.

Debates

How should 25-hydroxyvitamin D assays be standardised and thresholds defined?

Assay-to-assay variability in 25-hydroxyvitamin D measurement and disagreement over what constitutes sufficiency have driven standardisation efforts and continued discussion about how status should be defined and interpreted.

Key figures

• Michael F. Holick
• Roger Bouillon
• Munro Peacock

Seminal works

• holick-2011
• bouillon-2019

Frequently asked questions

Why is 25-hydroxyvitamin D, not the active form, used to check vitamin D status?

25-Hydroxyvitamin D is abundant and long-lived, so it reflects overall vitamin D supply, whereas the active 1,25-dihydroxy form is short-lived and tightly regulated and therefore does not track stores.

What is the difference between calcidiol and calcitriol?

Calcidiol (25-hydroxyvitamin D) is the hepatic storage and transport form used to assess status; calcitriol (1,25-dihydroxyvitamin D) is the kidney-produced active hormone that drives intestinal calcium and phosphate absorption.