Vitamin D is a secosteroid hormone.

The classification of vitamin D as a hormone rather than a traditional vitamin is supported by its molecular structure, synthesis pathway, and mechanism of action, which mirrors those of classical steroid hormones like testosterone and cortisol.

Mechanistic explanations

Vitamin D acts as a secosteroid hormone through several distinct molecular pathways:

• Genomic Signaling: The active form of vitamin D, calcitriol (1,25(OH)₂D), functions as a ligand for the vitamin D receptor (VDR). Upon binding, the VDR undergoes a conformational change and forms a heterodimer with the retinoid X receptor (RXR). This complex binds to specific DNA sequences known as vitamin D response elements (VDREs) to recruit co-regulator proteins (such as p300 or SRC-1) that remodel chromatin and initiate or suppress gene transcription.
• Non-Genomic Signaling: In addition to nuclear signaling, vitamin D can trigger rapid "non-genomic" responses through membrane-bound receptors. These pathways include the activation of second messenger systems, such as protein kinase C (PKC) and mitogen-activated protein kinases (MAPK), which occur within seconds to minutes—far faster than transcriptional changes.
• Steroid Cross-Talk: Vitamin D influences the broader steroid hormone environment by regulating the expression of enzymes such as CYP11A1, CYP17A1, and aromatase, which are critical for the biosynthesis of androgens and estrogens.

Clinical evidence

The widespread expression of the VDR ensures that vitamin D has systemic physiological effects across diverse tissues:

• Tissue-Wide Gene Expression: Transcriptomic research reveals that vitamin D regulates between 500 and 2,000 genes, representing approximately 3% to 5% of the human genome. VDR is found in classic target organs (bone, kidney, intestine) as well as the brain, skin, immune system, and prostate.
• Metabolic Regulation: In the kidney, VDR activation regulates genes such as Klotho and CYP24A1 to maintain calcium and phosphorus homeostasis. In immune cells, it modulates the expression of cathelicidin (an antimicrobial peptide), highlighting its role beyond mineral metabolism.
• Endogenous Synthesis: Unlike vitamins that must be obtained through diet, vitamin D is synthesized endogenously from 7-dehydrocholesterol in the skin via UV exposure, mirroring the synthesis of other cholesterol-derived steroids.

Bottom line

Vitamin D is a secosteroid hormone that functions as a ligand-activated transcription factor. Its active metabolite binds to the vitamin D receptor to directly modulate the expression of thousands of genes across virtually every tissue in the body, a process identical to the fundamental mechanism used by all steroid hormones.