Is Vitamin D a Hormone or a Vitamin?

December 31, 2025 •

3 min read

While commonly referred to as a vitamin, vitamin D is technically a prohormone—a substance the body converts into a hormone. This unique characteristic, where the body can produce most of its own supply from sun exposure, blurs the line between its classification as a simple nutrient and a complex endocrine regulator.

Quick Summary

Vitamin D is more accurately classified as a prohormone because the body can synthesize it endogenously and convert it into the active steroid hormone, calcitriol. This active form regulates critical physiological processes, including calcium and phosphate metabolism, much like other hormones, rather than functioning solely as a nutrient derived from diet.

Key Points

Not a True Vitamin: Unlike classic vitamins, vitamin D can be synthesized by the body when skin is exposed to UVB sunlight, so it does not meet the strict dietary definition of a vitamin.
A Prohormone: Vitamin D is more accurately described as a prohormone, a precursor that the body metabolizes into the active steroid hormone, calcitriol.
Synthesized from Cholesterol: The synthesis pathway starts with a cholesterol-based compound in the skin, which is a classic characteristic of steroid hormones.
Hormonal Action: The active form, calcitriol, binds to a nuclear receptor (VDR) found in cells throughout the body to regulate gene expression, a hallmark of hormonal function.
Systemic Effects: Its influence extends beyond bone health to modulate the immune system, brain function, and cell growth, illustrating a systemic hormonal role.
Endocrine Regulation: The conversion of inactive vitamin D to active calcitriol is tightly regulated by the body's endocrine system, maintaining precise calcium and phosphate balance.
Deficiency Consequences: A deficiency can lead to significant health problems like rickets in children and osteomalacia in adults, while excess can cause dangerous hypercalcemia.

What is a Vitamin vs. a Hormone?

To understand why the debate over vitamin D's classification exists, it is essential to first define what constitutes a vitamin and a hormone. A vitamin is an organic compound that an organism requires in small quantities for proper metabolic function but cannot synthesize itself and, therefore, must be obtained from the diet. A hormone, conversely, is a signaling molecule produced by glands and organs that travels through the bloodstream to regulate the function of distant cells or organs. The ability of humans to produce vitamin D endogenously from sun exposure distinguishes it from classic vitamins that are strictly reliant on dietary intake.

The Journey from Cholesterol to Calcitriol

The synthesis pathway of vitamin D strongly supports its classification as a prohormone. It begins in the skin, where a cholesterol precursor is converted into pre-vitamin D3 upon exposure to UVB radiation. This is then converted to vitamin D3 (cholecalciferol), which requires two metabolic steps to become the active steroid hormone, calcitriol.

Vitamin D3 is hydroxylated in the liver into 25-hydroxyvitamin D [25(OH)D], or calcidiol.
Calcidiol is then hydroxylated in the kidneys to become 1,25-dihydroxyvitamin D [1,25(OH)D], or calcitriol.

This two-step conversion is regulated by factors like parathyroid hormone and blood calcium, similar to other endocrine feedback loops. Its synthesis from cholesterol and conversion into an active molecule that regulates systemic processes supports its classification within the hormone category.

The Multifunctional Role of Vitamin D Receptors (VDRs)

The hormonal nature of vitamin D is further confirmed by its mechanism of action. The active form, calcitriol, binds to a vitamin D receptor (VDR) found in cells throughout the body. VDR is a nuclear receptor similar to those for other steroid hormones. When calcitriol binds to VDR, the complex influences gene expression in the cell nucleus. The widespread presence of VDRs and the ability to modulate hundreds of genes highlight vitamin D's broad physiological influence.

Vitamin D's wide-ranging effects include:

Bone Health: Regulates calcium and phosphate absorption and blood levels, essential for bone mineralization.
Immune System Modulation: Influences the immune response, helping control infections and reduce inflammation.
Brain and Neurological Function: Suggested link between levels and mood regulation.
Cell Growth and Proliferation: Modulates cell growth, with exploration into its potential role in preventing certain cancers.
Cardiovascular Health: Associated with cardiovascular regulation, and low levels linked to increased heart disease risk.

Comparison Table: Vitamin vs. Hormone


Feature	Classic Vitamin	Vitamin D (as a Hormone)
Source	Must be consumed through diet.	Primarily produced in the skin via sun exposure, with minor dietary intake.
Synthesis	Cannot be synthesized by the body.	Can be synthesized endogenously from a cholesterol precursor.
Mechanism	Acts as a coenzyme or antioxidant in metabolic processes.	Acts as a steroid hormone, binding to a specific nuclear receptor (VDR) to regulate gene expression.
Active Form	Often active upon absorption.	Is a prohormone that requires activation by the liver and kidneys to become its active hormonal form, calcitriol.
Regulation	Intake is regulated externally (diet).	Endogenous production and activation are tightly regulated by the body's endocrine system.
Function	Supports metabolic pathways and cellular health.	Functions as a systemic signaling molecule, influencing a wide range of physiological processes.

Conclusion: A Misnomer with Important Consequences

Scientific consensus classifies vitamin D as a prohormone converted to the active steroid hormone, calcitriol. Initially deemed a vitamin due to its role in preventing rickets via diet, its cholesterol synthesis and hormonal actions through VDRs reveal its true nature. This distinction is critical for health, given its broad effects. Supplements are important for those at high risk of deficiency, like older adults. Its hormonal function impacts numerous health aspects. For additional reading on vitamin D's function, refer to the National Institutes of Health website.

The Consequences of Deficiency and Toxicity

Both insufficient and excessive vitamin D levels carry significant health risks.

Deficiency

Bone issues: Causes rickets in children and osteomalacia/osteoporosis in adults.
Immune dysfunction: Can weaken the immune system.
Increased disease risk: Associated with higher likelihood of heart disease, certain cancers, and autoimmune conditions.

Toxicity (Hypervitaminosis D)

High calcium levels: Excessive intake results in hypercalcemia.
Hypercalcemia symptoms: Includes nausea, vomiting, confusion, and muscle weakness.
Calcification: Long-term excess can lead to soft tissue and blood vessel calcification.

Managing vitamin D levels, ideally with professional guidance, is essential due to its powerful effects.

Frequently Asked Questions

It was originally classified as a vitamin in the early 20th century because a deficiency was identified as the cause of rickets, a dietary disease. Though it’s now understood as a prohormone, the name has remained due to historical and traditional usage.

The active, hormonal form of vitamin D is calcitriol, or 1,25-dihydroxyvitamin D. The body synthesizes it from inactive vitamin D through two hydroxylation steps that occur in the liver and kidneys.

For most people, it is difficult to get enough vitamin D from food alone, as few foods naturally contain significant amounts. While fortified foods and supplements exist, the primary source for many is synthesis through sun exposure.

Yes, excessive intake of vitamin D from supplements can lead to toxicity, causing hypercalcemia, or high blood calcium levels. This is rare from food sources and impossible from sun exposure because the body self-regulates its production.

Vitamin D, in its active hormonal form, calcitriol, helps the body absorb calcium and phosphorus from the food you eat. This process is crucial for the mineralization of bones, ensuring they remain strong and healthy throughout life.

People in northern climates receive less intense ultraviolet B (UVB) radiation, especially during winter months. This reduces the skin's ability to synthesize vitamin D, making supplementation necessary to maintain adequate levels.

The best way to determine your vitamin D status is to have your blood levels measured by a doctor. A test for 25-hydroxyvitamin D is the most accurate way to assess sufficiency.