What is 25 hydroxy vitamin D also known as?

November 19, 2025 •

3 min read

The human body naturally converts vitamin D, whether from sunlight or diet, into a chemical known as 25 hydroxy vitamin D, or 25(OH)D. This metabolite is the most reliable indicator of your overall vitamin D status and is also commonly known as calcifediol or calcidiol.

Quick Summary

The metabolite 25-hydroxy vitamin D, commonly abbreviated as 25(OH)D, is clinically referred to as calcidiol or calcifediol. It is the principal circulating form of vitamin D, making its blood level the primary measure for assessing a person's total vitamin D supply from both sun exposure and dietary intake.

Key Points

Name Synonyms: 25 hydroxy vitamin D is also known as calcifediol or calcidiol.
Metabolic Pathway: It is the product of vitamin D's first hydroxylation step, which occurs primarily in the liver.
Clinical Marker: The serum 25(OH)D level is the best clinical indicator of a person's total vitamin D stores from all sources.
Storage Form: Calcifediol acts as the main circulating and storage form of vitamin D within the body due to its longer half-life compared to the active form.
Test Indicator: The 25-hydroxy vitamin D test is crucial for diagnosing vitamin D deficiency and insufficiency, which can affect bone health and other body systems.
Active Conversion: 25(OH)D is converted in the kidneys into the active hormone 1,25-dihydroxyvitamin D, or calcitriol.
Influencing Factors: Levels of 25(OH)D are influenced by sun exposure, dietary intake, age, BMI, genetics, and certain medical conditions or medications.

The answer to what is 25 hydroxy vitamin D also known as, involves its chemical names: calcifediol and calcidiol. Vitamin D from sun exposure or diet is converted in a two-step process to become active. The initial step occurs in the liver, transforming it into 25(OH)D. This is then sent to the kidneys for conversion into the active hormonal form, 1,25-dihydroxyvitamin D (calcitriol).

The Role of 25 Hydroxy Vitamin D in the Body

25(OH)D is crucial as a storage form of vitamin D with a half-life of two to three weeks, making its serum level the best measure of total body vitamin D stores. It's the primary precursor for the active form, calcitriol, which helps regulate calcium and phosphate for bone health.

The Importance of the 25-Hydroxy Vitamin D Test

The 25-hydroxy vitamin D test is vital for assessing vitamin D status. Those who may benefit from testing include:

Individuals with limited sun exposure.
Older adults.
People with obesity.
Those with conditions affecting nutrient absorption.
Patients with osteoporosis.
Breastfed infants.

Understanding Your 25(OH)D Test Results

Results are usually in ng/mL or nmol/L. Here's a general guide:


Health Status	25(OH)D Levels (ng/mL)	25(OH)D Levels (nmol/L)
Deficiency	Below 12	Below 30
Potential Inadequacy	12 to <20	30 to <50
Normal/Sufficient	20 to 50	50 to 125
High	Higher than 50	Higher than 125

Discuss your results with a healthcare provider due to potential variations in reference ranges.

How the Body Converts Vitamin D to 25(OH)D

Sunlight exposure on the skin produces vitamin D3 (cholecalciferol) from 7-dehydrocholesterol. Dietary intake provides vitamin D2 or D3. Both forms travel to the liver where enzymes like CYP2R1 convert them into 25-hydroxyvitamin D2 (ercalcidiol) and 25-hydroxyvitamin D3 (calcidiol), collectively measured as total 25(OH)D to assess vitamin D status.

Distinguishing 25(OH)D from 1,25(OH)2D

It's important to differentiate between 25-hydroxyvitamin D (calcifediol/calcidiol) and 1,25-dihydroxyvitamin D (calcitriol).

25-Hydroxyvitamin D (Calcifediol/Calcidiol): This is the storage form with a longer half-life, reflecting total vitamin D reserves. It's what standard tests measure.
1,25-Dihydroxyvitamin D (Calcitriol): This is the active hormonal form with a shorter half-life. Its levels are tightly regulated and not a good indicator of overall vitamin D stores. Calcitriol tests are used for specific issues like calcium regulation or kidney problems.

Factors Affecting 25(OH)D Levels

Individual 25(OH)D levels are influenced by several factors:

Sun Exposure: Affected by geography, season, and sunscreen use.
Weight: Vitamin D can be stored in fat tissue, potentially requiring higher intake for those with higher BMI.
Ethnicity: Darker skin with more melanin reduces vitamin D production from sunlight.
Age: Vitamin D synthesis from sun decreases with age.
Dietary Factors: Fat intake can influence vitamin D absorption.
Genetics: Genetic variations can impact vitamin D metabolism.

Conclusion

In conclusion, 25 hydroxy vitamin D is also known as calcifediol and calcidiol. It serves as the primary measure of the body's total vitamin D stores. Though not the active hormone, its conversion to calcitriol is vital for health, especially bone health. Monitoring 25(OH)D levels helps diagnose deficiency, and understanding the factors influencing these levels is key to maintaining sufficiency. The distinction between calcifediol (storage) and calcitriol (active) underscores why the 25(OH)D test is the standard for assessing nutritional vitamin D status.

Note: For further reading on vitamin D's broader physiological roles, the National Institutes of Health provides comprehensive fact sheets.

Frequently Asked Questions

25 hydroxy vitamin D (calcifediol) is the best indicator of overall vitamin D status because it is the main circulating and storage form of the vitamin with a long half-life. The active form, 1,25-dihydroxyvitamin D, has a much shorter half-life and is tightly regulated, meaning its levels can appear normal even when the body's vitamin D reserves are low.

The primary function of 25 hydroxy vitamin D is to serve as the body's main reservoir and transport form of vitamin D. It is a precursor that is later converted into the active hormonal form (calcitriol) needed for regulating calcium and phosphate levels in the body.

The most common and accepted synonyms for 25 hydroxy vitamin D are calcidiol and calcifediol.

25 hydroxy vitamin D levels are typically measured with a blood test. A healthcare provider draws a blood sample, and the lab analyzes the serum to determine the concentration of 25(OH)D, which reflects your vitamin D status.

Low levels of 25 hydroxy vitamin D indicate a vitamin D deficiency or insufficiency. This can be caused by inadequate sun exposure, poor diet, malabsorption, or certain medical conditions. Severe deficiency can lead to bone weakness, rickets in children, or osteomalacia in adults.

Yes, taking excessively high doses of vitamin D supplements over a prolonged period can lead to vitamin D intoxication, characterized by very high serum 25(OH)D levels (usually over 150 ng/mL). This can result in hypercalcemia and potentially serious health issues affecting the kidneys and liver.

Yes, dietary intake can affect your 25 hydroxy vitamin D levels, though sun exposure is the most significant source for most people. The vitamin D from foods (like fatty fish or fortified dairy) and supplements is processed by the liver to produce 25(OH)D.