Summary
| Direction in Long COVID | ↕ Inconsistent — reported as both higher and lower across studies, with no consistent direction |
|---|---|
| Category | Metabolic |
| Also described as | Calcidiol |
| Compared against | HC (healthy controls) |
| Associated symptoms | Immune modulation |
| Test type | Blood test (serum / plasma / whole blood) |
| LOINC code | 1989-3 |
| Source | Kallaste et al. 2025 — doi:10.3390/medicina61060996 |
| Condition atlas | Long COVID Biomarker Atlas |
Other conditions where Vitamin D (25-OH) is reported
- Vitamin D (25-OH) in Gulf War Illness — ↕ inconsistent vs HGV
See the cross-condition hub: Vitamin D (25-OH) across conditions.
Trials, interventions and tests
In clinical-trial registrations this marker maps to the outcome term Levels of vitamin D (5 registered trials use a matching outcome measure).
Clinical trials using Vitamin D (25-OH) as an outcome
- NCT07285707: Acupuncture and Chinese Herbal Medicine for Long COVID (Recruiting)
Outcome measure: Change in Laboratory Biomarkers - Vitamin D levels. - NCT05633472: The Roles of Vitamin D and Microbiome in Children With Post-acute COVID-19 Syndromes (PACS) and Long COVID (Completed)
Outcome measure: Levels of vitamin D - NCT05356936: Pilot Study of Vitamin K2 (MK-7) and Vitamin D3 Supplementation and the Effects on PASC Symptomatology and Inflammatory Biomarkers (Completed)
Outcome measure: Change in Vitamin D3 levels as measured by blood test - NCT04751669: Efficacy of a Dietary Supplementation in Reducing Hospital Admissions for COVID-19. Randomized Clinical Trial (Completed)
Outcome measure: Micronutrient basal status (Vitamin B1, Vitamin B6, 25-OH-Vitamin D and Folic Acid - NCT05630339: Magnesium and Vitamin D Combination for Post-COVID Syndrome (Completed)
Outcome measure: Change from Baseline Serum vitamin D levels at 4 months
Interventions studied alongside this marker
Hypothesis-generating links from trial outcome usage and literature co-occurrence. Not treatment recommendations.
- Vitamin D — 3 trials, 6 PubMed co-citations
e.g. Beyond the calendar: A narrative review on chronobiological drivers of prognosis in resectable NSCLC. - Acupuncture — 1 trial, 6 PubMed co-citations
e.g. Beyond Conventional Therapy: A Comprehensive Review of Herbal, Traditional, and Emerging Interventions for… - Vitamin K2 — 1 trial, 6 PubMed co-citations
e.g. Vitamins and Cancer Risk: A Comprehensive Review of Epidemiologic and Clinical Evidence.
Commercial test availability
Vitamin D, 25-Hydroxy — commercial laboratory test.
Links are for reference only; availability, specimen requirements and coverage vary by location.
Also sold as a nutrient or supplement
Vitamin D3 (cholecalciferol) (vitamin). 25-OH vitamin D is the measured metabolite; D3 is the common supplement form. This tag means the analyte is also available orally; it is not a product endorsement and a low or high level does not by itself justify supplementation.
Related Metabolic markers in Long COVID
- LDH (Lactate dehydrogenase) ↑
- Carnitine / Acylcarnitines ↕
- Lactate ↑
- 4-Aminobutanoate (GABA) ↑
- Xanthine ↑
- Thymine ↓
- Uracil ↓
- Porphobilinogen ↑
Full list: Long COVID Biomarker Atlas.
Frequently asked questions
Is Vitamin D (25-OH) high or low in Long COVID?
Mixed. Some studies report higher and others lower Vitamin D (25-OH) in Long COVID compared with healthy controls, so no single direction is established. Source: Kallaste et al. 2025.
What symptoms is Vitamin D (25-OH) associated with in Long COVID?
The cited literature associates this marker with Immune modulation. These are population-level associations and do not mean the marker causes the symptoms.
Which test measures Vitamin D (25-OH)?
It is measured as a blood test (serum / plasma / whole blood). The standard laboratory code is LOINC 1989-3. Commercial laboratories such as Quest Diagnostics, LabCorp, Ulta Lab Tests list an orderable assay.
Is one abnormal Vitamin D (25-OH) result diagnostic of Long COVID?
No. No single biomarker is diagnostic of Long COVID. Vitamin D (25-OH) findings come from group comparisons, overlap considerably between patients and controls, and vary with study design, timing and comparison group. A result should be interpreted by a clinician alongside symptoms, history and other tests.
Is Vitamin D (25-OH) specific to Long COVID?
No. The same marker is also reported in Gulf War Illness in this atlas, so it reflects shared biology (for example inflammation or vascular stress) rather than a condition-specific signature.
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Last reviewed: 2026-06-29 · Page generated from long-covid.json · Browse: Long COVID atlas · Biomarker Index · All marker pages
Disclaimer: This page is an educational synthesis of published, peer-reviewed research and is not medical advice. Biomarker findings vary across studies with case definitions, comparison groups, timing and assay methods. Direction of change reflects the predominant finding in the cited source and does not establish a diagnostic threshold. Discuss any test result with a qualified clinician.