What it is: The 25-hydroxyvitamin D (25-OH) test measures the circulating form of vitamin D used to assess vitamin D status. Vitamin D acts as a signaling molecule with receptors in many tissues.
Why it matters: Vitamin D status is common baseline context reviewed before peptide research across immune, hormonal, and tissue-repair topics. This is educational context, not a diagnosis.
Reference range: Public reference examples are described by guideline bodies (see below). Your lab's printed reference range takes priority.
Key thing to know: No fasting required; testing in late winter or early spring gives the most informative reading. Interpret with a licensed provider.
What is Vitamin D (25-OH)?
The test formally called "25-hydroxyvitamin D" or "25-OH Vitamin D" measures your body's stored form of vitamin D3, the version that circulates in the bloodstream and reflects your true vitamin D status. Despite the name "vitamin," vitamin D3 (cholecalciferol) functions more accurately as a steroid prohormone. It binds to the vitamin D receptor (VDR), which is expressed in nearly every cell in the body, including immune cells, skeletal muscle, brain, and gonads, making it one of the most broadly acting signaling molecules in human physiology.
Vitamin D status is frequently reviewed as baseline context before peptide research because vitamin D signaling intersects with immune, hormonal, and tissue-repair biology described in the research literature. This page is educational context about what the 25-OH test measures. It is not a diagnosis, a determination about any individual, or a claim about any outcome.
Public reference examples
The categories below are public reference examples described by the Food and Nutrition Board at the National Academies and summarized by the NIH Office of Dietary Supplements. They are shown for educational context only and are not a determination about any individual. Your lab's printed reference range takes priority.
These are public reference examples, not personal interpretation. Vitamin D testing uses 25-OH Vitamin D specifically, not 1,25-OH (calcitriol). Your lab's printed reference range takes priority, and results should be reviewed with a licensed provider.
Why this marker is commonly reviewed
Vitamin D is often reviewed as baseline context across several peptide research areas. The notes below describe where it appears in the research literature and are for educational context only, to be interpreted with a licensed provider. They are not claims about any effect on any individual.
Immune research: Vitamin D signaling is discussed in immune research alongside the endogenous antimicrobial peptide LL-37, which is itself a research compound. Researchers reviewing immune-focused peptides such as Thymosin Alpha-1 often note baseline vitamin D status as part of the research context.
Hormonal research: Vitamin D receptors are described in many tissues, and vitamin D status appears in hormonal research as one baseline variable. Researchers reviewing GH-axis peptides such as Ipamorelin or CJC-1295 often record vitamin D status as part of a baseline panel.
Tissue repair research: Vitamin D is discussed in relation to inflammatory-resolution signaling in the research literature. Repair-focused research compounds such as BPC-157 and TB-500 are studied within overlapping inflammatory pathways, so vitamin D status is sometimes noted as baseline context.
How to get this test.
Where to order
Standard blood draw at LabCorp, Quest Diagnostics, or through a physician. Also available via direct-to-consumer services without a prescription. Frequently included in comprehensive wellness panels and annual physicals.
How to prepare
No fasting required. Can be drawn any time of day. Test in late winter or early spring for the most clinically informative reading, this is when levels are at their annual low after months of reduced sun exposure.
What to ask for
"25-OH Vitamin D" or "Vitamin D, 25-Hydroxy", specify 25-OH, not 1,25-OH (calcitriol). Order alongside testosterone, cortisol, and hs-CRP to build a foundational hormonal and inflammatory baseline.
Peptides commonly researched in connection with this marker.
Goals where this biomarker is most relevant.
Related biomarkers
Vitamin D is often reviewed alongside these markers as part of a baseline panel. These links are provided for educational context.
Go deeper
How vitamin D fits into a complete longevity biomarker panel.
Read Research →Citations
- National Institutes of Health, Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. Serum 25-hydroxyvitamin D reference categories from the Food and Nutrition Board, National Academies. ods.od.nih.gov
- MedlinePlus, U.S. National Library of Medicine. Vitamin D Test. Notes that result interpretation should be discussed with a provider. medlineplus.gov