Vitamin D is a steroid hormone with a misleading name.

Vitamins, by definition, are compounds your body cannot make on its own (which is why you have to eat them). But your skin synthesizes vitamin D whenever UVB sunlight hits it.

When researchers worked out Vitamin D’s full pathway in the 1970s, they found it binds a nuclear receptor that regulates roughly 1,000 to 2,000 genes (about 5 to 10% of the human genome). That receptor sits in the same family as the receptors for estrogen, testosterone, cortisol, and thyroid hormone.

This reframing explains why a “vitamin” reaches into immunity, blood pressure, glucose control, and (as the TARGET-D trial suggested) heart attack risk. In the rest of this post, we’ll trace how vitamin D became a hormone in everything but name, what the vitamin D receptor actually does, and what the hormone framing means for testing and supplementation.

Why vitamin D was originally misclassified as a vitamin

Vitamin D got its name in 1922. Elmer McCollum had spent the previous decade naming fat-soluble compounds in cod liver oil by letter (vitamin A, then B, then C), and the factor that cured rickets in rats became the fourth letter. The label stuck before anyone understood what the molecule did or where it came from. (Elmer McCollum became known as “Dr Vitamin” and famloulsly said, “Eat what you want after you have eaten what you should.”)

How vitamin D becomes an active hormone

What your skin produces (or what arrives in food) is not the active form. Vitamin D₃ is a precursor that has to be activated in two steps:

1. The liver adds a hydroxyl group, producing 25-hydroxyvitamin D (25(OH)D, calcidiol). This is the form measured by a standard vitamin D blood test, because it has a half-life of weeks and reflects overall stores.
2. The kidney adds a second hydroxyl, producing 1,25-dihydroxyvitamin D (calcitriol). Calcitriol is the biologically active hormone. Calcitriol’s half-life is hours, and its blood levels are tightly regulated by parathyroid hormone and serum calcium.

Vitamin D₃ from skin or diet is activated through hydroxylation in the liver and kidney before binding to the vitamin D receptor.

That two-step activation in distant endocrine organs is characteristic of a hormone. The kidney functions as an endocrine gland, and calcitriol is its product. The 25(OH)D level on a blood test is closer to a steroid pre-hormone reading (like checking pregnenolone) than to a nutrient status marker.

The vitamin D receptor (VDR)

Once activated, calcitriol travels through the blood and binds to the vitamin D receptor (VDR), a protein that lives in the nucleus of nearly every cell type studied: intestine, kidney, bone, parathyroid, pancreatic beta cells, T and B lymphocytes, vascular smooth muscle, endothelium, heart muscle, and many neurons.

The VDR belongs to the nuclear receptor superfamily, the same family that contains the receptors for:

• Estrogen
• Testosterone
• Cortisol
• Thyroid hormone
• Retinoic acid (the active form of vitamin A)

Each of these receptors works the same way. A hormone binds, the receptor changes shape, pairs up with a partner protein, slides into DNA at specific response elements, and turns genes on or off. VDR partners with the retinoid X receptor (RXR), then regulates transcription of an estimated 1,000 to 2,000 genes.

The vitamin D receptor (VDR) sits in the same superfamily as the receptors for estrogen, testosterone, cortisol, thyroid hormone, and retinoic acid. All bind a ligand, dock to specific DNA sequences, and switch genes on or off.

This is why vitamin D effects span so many systems: it’s a transcription factor running in parallel across most tissues at once.

How vitamin D drives different organ systems

The hormone framing predicts that vitamin D should reach well beyond bone. Vitmain D has affects across cardiovascular, autoimmune, respiratory, cancer, falls (obviously has some relation to bone health), and diabetes:

Body system	Effect	Study
Cardiovascular	52% lower risk of repeat heart attack	TARGET-D, post-MI patients held at 40 to 80 ng/mL
Autoimmune	22% lower rate of new autoimmune disease (RA, psoriasis, autoimmune thyroid, and others)	VITAL, 25,871 adults, 2,000 IU/day, 5 years
Respiratory infections	12% fewer acute respiratory infections overall; 70% fewer in people starting below 10 ng/mL	IPD meta-analysis of 25 RCTs, 10,933 participants
Cancer mortality	25% lower cancer mortality after excluding the first 2 years; 17% across the full window	VITAL
Falls	22% fewer falls in adults who started deficient	Pooled meta-analysis
Type 2 diabetes	12% lower progression to T2D (non-significant); larger in those who reached optimal blood levels	D2d, 2,423 adults with prediabetes, 4,000 IU/day

Why viewing Vitamin D as a hormone matters for testing and supplementation

The hormone vs vitamin framing has two practical consequences for testing and supplementation:

Optimal blood levels are not the same as minimum levels. The original “vitamin” framing produced an RDA (600 to 800 IU per day) tuned to prevent rickets, which is the most severe deficiency disease. The hormone framing asks a different question: at what 25(OH)D level does the receptor have enough substrate to function in all the tissues that express it? The Endocrine Society suggests 30 to 50 ng/mL. The TARGET-D heart trial used 40 to 80 ng/mL. Both are well above the deficiency threshold of 20 ng/mL.

Overdose risk is real, because it is a steroid hormone. Water-soluble vitamins like vitamin C get flushed in urine. Steroid hormones do not. Sustained intake above ~10,000 IU per day can drive calcium absorption past what bone and the kidney can handle, leading to hypercalcemia. This is rare in practice, but it is a reminder that the “more is better” rule of thumb for B vitamins does not apply here.

How can you measure Vitamin D?

If you want to know where you stand, the 25-hydroxyvitamin D blood test is the standard measurement, and it is included in the comprehensive panel of an Empirical Health membership.