Vitamin D3: Who Should Be Tested, and How Much to Take

What vitamin D is and how you get it

Vitamin D is a hormone precursor rather than a vitamin in the ordinary sense. Skin makes it from cholesterol when exposed to UVB light; it then converts first in the liver to 25-hydroxyvitamin D — the stored form and the form measured in blood — and then in the kidney to the active form, which is regulated tightly and is not what a supplement raises.

Few foods naturally contain meaningful vitamin D: oily fish and fish liver oil chiefly, with some fortified milk and cereals. For most people, sun exposure and a supplement together determine status, which is why deficiency is common at higher latitudes, in winter, in darker skin, in older adults, in people who are indoors most of the day or cover their skin, and in people with obesity, whose vitamin D is sequestered in fat tissue.

Why routine testing stopped being the default

Observational studies consistently linked low 25-hydroxyvitamin D levels with fractures, cardiovascular disease, diabetes, depression, cancer, and infection. The problem is that low levels track with indoor lifestyles, darker skin, obesity, and chronic illness — so the association is not proof that raising the level fixes the outcome.

Large randomized trials of supplementation in generally healthy people have largely not shown reductions in fractures, cardiovascular events, or cancer in the overall study populations. The 2024 Endocrine Society clinical practice guideline therefore suggests against routine 25-hydroxyvitamin D screening in healthy adults who have no specific indication, while still recommending sensible supplementation.

What the 2024 guideline actually recommends

The guideline, published in the Journal of Clinical Endocrinology & Metabolism, updates and replaces the 2011 vitamin D deficiency guideline. Its essential points, as summarized by the Endocrine Society: for children and adolescents, routine supplementation of 600 to 1,000 IU daily depending on age; for adults under 50, 400 to 1,000 IU daily or dietary sources; for adults 50 to 74, 1,000 to 2,000 IU daily; and for adults 75 and older, 1,000 to 2,000 IU daily with testing and treatment suggested in that age group.

It also suggests against testing generally healthy adults for deficiency, and specifically against routinely testing adults with dark skin complexes, adults with obesity, and generally healthy adults — because in those groups, a low number has not been shown to predict benefit from treatment. Where testing is appropriate is in people with established indications: osteoporosis, malabsorption, chronic kidney disease, granulomatous disease, or medications that affect vitamin D metabolism.

The guideline also addresses prediabetes: it suggests vitamin D supplementation for adults with prediabetes, as the evidence there does show a reduction in progression to diabetes.

D3 versus D2, and why megadosing backfires

Supplements come as D2 (ergocalciferol) and D3 (cholecalciferol). D3 is the form skin produces and is generally more effective at raising and sustaining 25-hydroxyvitamin D, which is why it is the form most clinicians prescribe; D3 injections are also offered in clinics for patients who prefer them.

More is not better. Very high intermittent doses have produced worse outcomes in some trials, including increased falls in older women given large annual boluses, and excessive vitamin D can raise blood calcium enough to cause nausea, weakness, constipation, and kidney stones. A steady, moderate daily dose is the approach the guideline's numbers describe, and it is far more predictable than a huge monthly dose.

If you are taking a supplement and a level comes back low, ask whether something is driving it — weight, medications, malabsorption, minimal sun — rather than simply escalating the dose.

What Vitality Modern Medicine offers

In clinic, a vitamin D3 booster injection is available at $30, and it is one of the boosters included in the testosterone therapy program. IV infusions start at $129 and can be customized with boosters. Each is given after a brief health screening and a conversation about your goals.

For most patients, a daily oral supplement at guideline-range doses is the simpler route, and a level is only worth drawing when there is a specific reason. If fatigue or bone health is the concern, those are evaluated as part of a hormone-care workup rather than assumed to be a vitamin D problem.

Prices shown are current clinic prices as of October 2026, are cash-pay, and can change. A provider decides whether a specific injection or supplement is appropriate for you before anything is administered.

This article summarizes published research for general education. It is not medical advice, does not recommend a specific product or dose, and does not replace evaluation by your own provider.