Is Vitamin D2 Known as Calcitriol? Understanding the Key Differences

January 12, 2026 •

4 min read

According to the National Institutes of Health, vitamin D must undergo two hydroxylation steps in the body to become active. Thus, the answer to "is vitamin D2 known as calcitriol?" is definitively no; they are distinct compounds serving different roles in vitamin D metabolism.

Quick Summary

Vitamin D2 (ergocalciferol) is an inactive precursor found in plants and yeast. Calcitriol is the body's active hormonal form of vitamin D, produced primarily in the kidneys through a metabolic process involving D2 or D3.

Key Points

Not the Same: Vitamin D2 (ergocalciferol) is not calcitriol; it is an inactive precursor that the body must metabolize.
The Active Form: Calcitriol is the biologically active, hormonal form of vitamin D, produced primarily in the kidneys.
Metabolic Process: Both Vitamin D2 and D3 are converted in the liver to 25(OH)D and then in the kidneys to calcitriol.
Different Potency and Use: D2 is a dietary supplement, while calcitriol is a potent, fast-acting prescription medication for specific conditions like kidney disease.
Risk of Hypercalcemia: Due to its high potency, calcitriol carries a higher risk of raising blood calcium levels excessively, requiring careful medical supervision.
Sources: D2 comes from plants and fortified foods, whereas calcitriol is produced internally.

What is Vitamin D2 (Ergocalciferol)?

Vitamin D2, known as ergocalciferol, is a form of vitamin D sourced from plants and fungi, such as mushrooms and yeast. This is also the version of vitamin D used to fortify foods like cereals and plant-based milks. As a precursor, ergocalciferol is biologically inert, meaning it has no significant effect in the body until it is converted into a usable form.

When ingested, vitamin D2 is first sent to the liver, where it is converted into 25-hydroxyvitamin D2, also known as ercalcidiol. This metabolite then circulates in the blood and is eventually transported to the kidneys for the final conversion step into the active hormone, calcitriol.

Key characteristics of Vitamin D2:

Source: Plant-based and fungi (e.g., mushrooms).
Potency: Considered less potent than vitamin D3 in raising serum vitamin D levels.
Metabolism: Requires a two-step activation process in the liver and kidneys.
Use: Used in dietary supplements and food fortification.

What is Calcitriol?

Calcitriol is the biologically active, hormonal form of vitamin D. It is produced by the kidneys from the metabolites of both vitamin D2 and vitamin D3. Its primary function is to regulate calcium and phosphate levels in the body, playing a crucial role in bone mineralization and overall skeletal health. Calcitriol is also known as 1,25-dihydroxycholecalciferol or 1,25(OH)2D3 when derived from vitamin D3.

Because it is already in its active form, calcitriol acts much more quickly than vitamin D2 or D3. Due to its potency, calcitriol is typically prescribed by a physician for individuals with specific medical conditions that affect their ability to produce the active hormone, such as chronic kidney disease or hypoparathyroidism.

The crucial functions of calcitriol include:

Increasing Calcium Absorption: Enhances the uptake of dietary calcium from the gastrointestinal tract.
Regulating Parathyroid Hormone (PTH): Works with PTH to maintain calcium homeostasis.
Promoting Bone Remodeling: Stimulates the release of calcium from bone when blood levels are low, a process carefully balanced by other mechanisms.

The Vitamin D Activation Pathway: From Precursor to Hormone

The metabolic journey of vitamin D, whether it starts as D2 or D3, is a multi-step process crucial for understanding why vitamin D2 and calcitriol are not the same.

Ingestion or Synthesis: Vitamin D2 (from food/supplements) or Vitamin D3 (from sunlight or animal sources) enters the body.
First Hydroxylation in the Liver: The liver converts the vitamin D precursor into 25-hydroxyvitamin D [25(OH)D], also known as calcidiol. This is the major circulating form of vitamin D and what is measured in blood tests.
Second Hydroxylation in the Kidneys: The kidneys convert calcidiol into the active hormone, 1,25-dihydroxyvitamin D, or calcitriol. This final conversion is tightly regulated to ensure proper calcium levels.

This pathway illustrates that calcitriol is the end product of a metabolic process that begins with inactive precursors like vitamin D2. For more detailed information on the vitamin D pathway, consult the National Institutes of Health's fact sheet.

Comparison: Vitamin D2 vs. Calcitriol


Feature	Vitamin D2 (Ergocalciferol)	Calcitriol (1,25-dihydroxyvitamin D)
Classification	Inactive precursor	Biologically active hormone
Source	Plants, fungi, fortified foods	Produced in the body (primarily kidneys)
Medical Use	Dietary supplement for preventing/treating vitamin D deficiency	Prescription medication for kidney disease or hypoparathyroidism
Absorption	Absorbed from the gut, requires metabolism	Rapidly absorbed and immediately active
Metabolism	Requires two conversions (liver, then kidneys)	The end-stage, active form; no further conversion needed
Potency	Less effective at raising vitamin D levels than D3	Highly potent, direct effect
Risk of Toxicity	Lower risk of hypercalcemia in chronic use compared to calcitriol	Higher risk of hypercalcemia, requires careful monitoring

Conclusion: Clarifying the Relationship

To answer the question, "Is vitamin D2 known as calcitriol?", the answer is a clear no. Vitamin D2 is the inactive precursor, ergocalciferol, while calcitriol is the end-stage, active hormonal form. This distinction is critical for anyone managing vitamin D levels, especially those with underlying health conditions. While supplements with vitamin D2 are effective for general deficiency, calcitriol is a powerful, fast-acting medication reserved for specific medical needs. For the average person, focusing on maintaining adequate levels of either vitamin D2 or D3 through diet, sun, and supplementation is the correct approach, letting the body naturally produce the necessary calcitriol.

Key distinctions to remember:

D2 is a raw material; calcitriol is the finished product. The body processes D2 (or D3) to create the active hormone calcitriol.
Differing medical uses. Vitamin D2 is used for general supplementation, while calcitriol is for specific medical treatments.
Activation is key. The metabolic conversion in the liver and kidneys is what makes vitamin D biologically active, turning an inactive precursor into a potent hormone.

Frequently Asked Questions

Vitamin D2 (ergocalciferol) is an inactive precursor derived from plant sources. Calcitriol is the active, hormonal form of vitamin D, produced by the kidneys, that regulates calcium in the body.

Yes, calcitriol is the biologically active form of vitamin D. The body converts inactive forms like D2 and D3 into calcitriol through a two-step process involving the liver and kidneys.

Calcitriol is produced in the kidneys. It is synthesized from 25-hydroxyvitamin D (calcidiol), which is first created in the liver from dietary or sun-synthesized vitamin D (D2 or D3).

No, calcitriol is not available from food or standard supplements. Precursor forms like vitamin D2 (from plants) or D3 (from animals/sunlight) are what the body metabolizes into calcitriol.

Calcitriol is a prescription medication for those who cannot properly metabolize inactive vitamin D into its active form. This is often the case with chronic kidney disease, where kidney function is impaired.

As an inactive precursor, the main role of vitamin D2 is to be a substrate for the body's vitamin D synthesis pathway. It must be metabolized by the liver and kidneys to become the active hormone calcitriol.

Yes, like vitamin D2, vitamin D3 (cholecalciferol) is also an inactive precursor. It is synthesized in the skin from sunlight or obtained from animal sources and must be metabolized into calcitriol.

Calcitriol's primary function is to regulate calcium and phosphate levels. It increases the absorption of calcium from food in the gut and plays a role in bone health.