Which Vitamin is Considered to be a Hormone?

January 11, 2026 •

4 min read

While commonly referred to as a vitamin, research from the National Institutes of Health and others indicates that vitamin D is functionally a prohormone. This is because unlike other vitamins, which the body cannot produce, vitamin D can be synthesized endogenously when skin is exposed to sunlight.

Quick Summary

The fat-soluble nutrient known as Vitamin D functions as a prohormone, a substance the body converts into a hormone. After sun exposure triggers synthesis in the skin, it is converted by the liver and kidneys into its active hormonal form, calcitriol, which regulates critical processes like calcium absorption and immune response.

Key Points

Prohormone, not just a Vitamin: Unlike true vitamins, the body can produce Vitamin D, classifying it as a prohormone that is converted into a steroid hormone.
Multi-organ Activation: The initial form of vitamin D is converted into its active hormonal state, calcitriol, through hydroxylation steps that occur in both the liver and the kidneys.
Widespread Receptor Presence: The active hormone, calcitriol, binds to widespread vitamin D receptors (VDR) found in cells throughout the body, enabling its broad systemic effects.
Regulates Calcium Homeostasis: As a hormone, calcitriol's most important function is regulating blood calcium and phosphorus levels, which is crucial for bone mineralization.
Influences Immune Function: Calcitriol acts as an immunomodulator, suppressing inflammation and regulating both the innate and adaptive immune systems.
Impacts Cell Growth and Differentiation: The hormone plays a role in cellular growth and differentiation, a key area of research for potential anti-cancer effects.
Deficiency is Common: A significant portion of the population suffers from vitamin D deficiency due to limited sun exposure, impacting overall health.
Supplementation May Be Necessary: For many, especially at-risk groups, dietary intake and supplements are required to maintain adequate levels due to insufficient sun synthesis.

From Nutrient to Endocrine Regulator

Vitamin D is a unique molecule that challenges the traditional definition of a vitamin. True vitamins are compounds that the body cannot synthesize and must be obtained from the diet. However, Vitamin D can be produced in the skin upon exposure to ultraviolet B (UVB) radiation from sunlight, qualifying it instead as a prohormone. Its hormonal status is further cemented by its complex metabolism and its function in regulating physiological processes far beyond its classic role in bone health.

The Multi-Step Activation of Vitamin D

The process that transforms inactive vitamin D into its potent, hormonal form, calcitriol, involves a sophisticated two-step hydroxylation process. This activation sequence, regulated by other hormones and minerals in the body, is characteristic of an endocrine system.

Synthesis in the Skin: When UVB light hits the skin, a cholesterol precursor called 7-dehydrocholesterol is converted into cholecalciferol, or Vitamin D3.
First Hydroxylation in the Liver: Cholecalciferol is then transported to the liver, where it is hydroxylated into 25-hydroxyvitamin D (25(OH)D), also known as calcidiol. Calcidiol is the primary form measured in blood to determine a person's vitamin D status.
Final Hydroxylation in the Kidneys: In the kidneys, a final hydroxylation step converts calcidiol into the biologically active and most potent form, 1,25-dihydroxyvitamin D, or calcitriol. This is the final hormone that carries out the major functions attributed to vitamin D.

Endocrine Functions of Calcitriol

As the active hormone, calcitriol exerts its effects by binding to the vitamin D receptor (VDR), a nuclear receptor found in cells throughout the body. This widespread presence of VDR, far beyond just bone tissue, explains the diverse and systemic impact of vitamin D.

Key functions of calcitriol include:

Calcium and Phosphate Homeostasis: This is the most well-known hormonal role. Calcitriol increases calcium absorption from the intestines, mobilizes calcium from bones when needed, and promotes calcium reabsorption in the kidneys, all to maintain stable blood calcium levels.
Immune System Regulation: The presence of VDR on immune cells like T-cells, B-cells, and macrophages allows calcitriol to modulate immune responses. It helps suppress overly aggressive inflammatory reactions, boosts innate immunity, and helps prevent autoimmunity.
Cell Growth and Differentiation: Calcitriol has been shown to inhibit cell proliferation and promote cell differentiation, with significant research focusing on its potential anti-cancer properties in various tissues like the breast, prostate, and colon.
Influence on Other Hormones: Calcitriol also provides feedback to the parathyroid glands, suppressing the release of parathyroid hormone (PTH), which plays a direct role in calcium regulation.

Comparison Table: Vitamin D vs. True Vitamins


Feature	Vitamin D (as a Prohormone)	True Vitamins (e.g., Vitamin C)
Origin	Can be synthesized in the body via sun exposure; also from diet.	Cannot be synthesized by the body and must be obtained from diet.
Active Form	Converted by organs (liver and kidneys) into its active hormonal form, calcitriol.	Typically active upon absorption or with minimal modification.
Mechanism	Binds to widespread nuclear receptors (VDR) to regulate gene expression.	Act as coenzymes or antioxidants in specific metabolic pathways.
Systemic Effect	Systemic, influencing a wide range of organs and physiological systems.	Primarily cellular, affecting local biochemical reactions.
Example Function	Regulates blood calcium levels and immune function.	Acts as an antioxidant, supports collagen synthesis.

Widespread Vitamin D Deficiency

Despite its vital, hormone-like functions, vitamin D deficiency is a major public health issue affecting a large portion of the global population. While sunshine is the primary source, modern lifestyles, increased indoor time, and geographic location all contribute to insufficient UVB exposure. This widespread deficiency has spurred extensive research into its link with various chronic diseases, though some studies have yielded mixed results.

The Importance of Supplementation

For those with limited sun exposure or certain health conditions, diet and supplements become the primary means of maintaining adequate vitamin D levels. Experts recommend supplements for at-risk groups, such as the elderly, individuals with darker skin, and those with specific medical conditions. Taking supplements with fatty meals can also enhance absorption, as vitamin D is a fat-soluble nutrient. However, excessive intake can lead to toxicity, causing dangerously high calcium levels in the blood.

Conclusion

Vitamin D's identity as a powerful, steroid-derived prohormone is crucial to understanding its systemic effects on the body. Its synthesis in the skin, multi-step activation by the liver and kidneys, and interaction with widespread nuclear receptors distinguish it significantly from traditional vitamins. This hormonal functionality allows it to play a central role in maintaining calcium balance, modulating the immune system, and influencing cellular growth. Given the high prevalence of deficiency, particularly in populations with limited sun exposure, recognizing vitamin D's hormonal nature is essential for informed discussions about public health, supplementation, and its complex link to chronic disease prevention.

Visit the National Institutes of Health for a comprehensive resource on Vitamin D.

Frequently Asked Questions

The main difference is that the body can produce its own vitamin D when exposed to sunlight, which is not true for other vitamins. This, along with its activation and function via widespread receptors to regulate systemic processes, classifies it as a prohormone rather than a simple nutrient.

The process begins with synthesis in the skin from sun exposure. The resulting cholecalciferol (D3) is then sent to the liver for a first conversion, and finally to the kidneys, where it is converted into the active hormone, calcitriol.

It is called this because sun exposure is the body's primary and most efficient way of producing it. When ultraviolet B (UVB) rays from the sun hit the skin, it triggers the synthesis of vitamin D3.

A deficiency can lead to a range of health issues, including low blood calcium levels, muscle weakness, bone pain (osteomalacia), and in children, bone deformities known as rickets. It is also linked to impaired immune function.

No, it is not possible to get too much vitamin D from sunlight alone. The skin regulates the amount it produces. Vitamin D toxicity is almost always the result of consuming excessive amounts of supplements.

Vitamin D receptors are present on various immune cells. The active hormone, calcitriol, binds to these receptors to modulate immune responses, helping to suppress autoimmune reactions and boost innate immunity.

Its hormonal form, calcitriol, is a key regulator of calcium. It increases the absorption of calcium from the gut, helps control its concentration in the blood, and works with parathyroid hormone to manage bone resorption and formation.