What is the name of the active form of vitamin D?

October 26, 2025 •

3 min read

The vitamin D you get from sunlight or dietary supplements is not biologically active until your body performs a two-step conversion process. The resulting hormone plays a vital role in regulating calcium and phosphate levels. So, what is the name of the active form of vitamin D, and how is it created?

Quick Summary

The active form of vitamin D is called calcitriol, or 1,25-dihydroxyvitamin D. It is synthesized through metabolic conversions in the liver and kidneys, acting as a crucial hormone for mineral balance and bone health.

Key Points

Calcitriol is the active form: The biologically active form of vitamin D is called calcitriol, or 1,25-dihydroxyvitamin D.
Activation is a two-step process: The conversion of inert vitamin D into active calcitriol requires two hydroxylation steps, one in the liver and a final one in the kidneys.
The kidneys play a crucial role: The kidneys are the primary site for the final activation of vitamin D, converting calcifediol into calcitriol.
Calcitriol regulates calcium: The main function of active vitamin D is to regulate blood calcium and phosphate levels by enhancing intestinal absorption.
Active vitamin D is a hormone: Once activated, calcitriol acts like a steroid hormone, binding to cellular receptors to regulate gene expression.
Deficiency affects the skeleton: Inadequate production or activation of vitamin D can lead to bone disorders such as rickets and osteomalacia.

The Metabolic Pathway to Active Vitamin D

Vitamin D is a fat-soluble secosteroid that acts as a prohormone, requiring conversion to its active form to function. This process involves the liver and kidneys. Vitamin D3 (cholecalciferol) from skin synthesis or vitamin D2 (ergocalciferol) from plants and supplements are initial, inert forms that must be metabolized.

Step 1: The Liver’s Role in Activation

Vitamin D travels to the liver for the first hydroxylation step, primarily by the enzyme CYP2R1, adding a hydroxyl group at the 25th carbon position. This yields 25-hydroxyvitamin D, or calcifediol (calcidiol). Calcifediol is the main circulating form and is measured in blood tests for vitamin D status. This hepatic conversion is largely unregulated.

Step 2: The Kidney’s Role in Final Activation

Calcifediol goes to the kidneys for the final, regulated activation. The enzyme 1-alpha-hydroxylase (CYP27B1) in the proximal tubules adds another hydroxyl group at the 1-alpha position. This creates 1,25-dihydroxyvitamin D, known as calcitriol, the active hormone. Renal calcitriol production is tightly controlled by factors like parathyroid hormone (PTH), FGF23, and calcium/phosphate levels.

Functions of Calcitriol: The Active Form of Vitamin D

Calcitriol acts as a steroid hormone, binding to the vitamin D receptor (VDR) in various tissues. This regulates hundreds of genes. Key functions include:

Regulating Calcium and Phosphate Homeostasis: Calcitriol increases intestinal absorption of calcium and phosphate and promotes kidney reabsorption. It also helps release calcium from bone with PTH when blood calcium is low.
Supporting Bone Health: Proper calcium and phosphate levels maintained by calcitriol are vital for bone mineralization. Deficiency can cause rickets and osteomalacia.
Modulating the Immune System: Calcitriol affects immune cells, regulating innate and adaptive responses and potentially reducing autoimmune disease risk.
Influencing Cell Growth and Differentiation: Calcitriol can inhibit cell proliferation and promote differentiation.
Other Roles: Research suggests roles in muscle function, brain development, and cardiovascular health.

Comparison of Key Vitamin D Metabolites


Feature	Calcifediol (25-hydroxyvitamin D)	Calcitriol (1,25-dihydroxyvitamin D)
Synonyms	Calcidiol, 25(OH)D	1,25-dihydroxycholecalciferol, 1,25(OH)2D
Produced In	Primarily the liver	Primarily the kidneys
Biological Activity	Biologically inactive precursor	Biologically active hormone
Measurement	Standard clinical marker for overall vitamin D status	Rarely measured unless kidney disease or other specific conditions are suspected
Half-Life	Weeks to months	Hours
Regulation of Synthesis	Largely unregulated; depends on substrate availability	Tightly regulated by PTH, FGF23, calcium, and phosphate levels

The Importance of the Kidney

The kidneys are crucial for producing active vitamin D. Impaired kidney function, as in chronic kidney disease, compromises calcitriol activation. This can lead to low calcitriol levels, disrupting calcium and phosphorus regulation and causing bone diseases like renal osteodystrophy. In these cases, activated vitamin D analogs may be given directly. The kidney's precise regulation of this final step is vital for mineral balance, highlighting its role as an endocrine organ.

For more information on vitamin D, refer to the Linus Pauling Institute.

Conclusion

The active, hormonal form of vitamin D is calcitriol, produced via a two-step pathway in the liver and kidneys. This compound is essential for mineral homeostasis and affects many physiological processes, including bone health and immune function. Understanding this conversion is key to appreciating vitamin D's complex role in health.

Frequently Asked Questions

Calcitriol is the hormonal, active form of vitamin D, also known as 1,25-dihydroxyvitamin D. It is primarily produced in the kidneys and regulates calcium and phosphate in the body.

The activation of vitamin D involves a two-step process. First, the liver converts vitamin D (from sun or diet) into 25-hydroxyvitamin D (calcifediol). Then, the kidneys convert calcifediol into the active form, calcitriol.

Doctors test for 25-hydroxyvitamin D (calcifediol) because it is the main circulating form of vitamin D and provides a more accurate reflection of the body's overall vitamin D status. Calcitriol levels are tightly regulated and have a very short half-life.

The main functions of active vitamin D, or calcitriol, include promoting calcium and phosphate absorption in the intestines, supporting healthy bone mineralization, and modulating the immune system.

The kidneys house the enzyme 1-alpha-hydroxylase, which performs the final conversion of the precursor 25-hydroxyvitamin D into the active hormone, calcitriol. Without proper kidney function, this activation step is compromised.

Yes, both vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol) are converted into their respective active forms, 1,25-dihydroxyvitamin D2 and 1,25-dihydroxyvitamin D3. Both are biologically active and bind to the vitamin D receptor.

Yes. While vitamin D intake is necessary, excessive amounts can lead to toxicity, causing hypercalcemia (abnormally high calcium levels) which can damage the heart, blood vessels, and kidneys.