Vitamin D25-hydroxyvitamin D · 25(OH)D
The 'sunshine vitamin', really a hormone, and a very common deficiency.
Vitamin D is unusual among vitamins in being one the body makes for itself, in skin exposed to UVB sunlight, with food and supplements as secondary sources. It is then converted by the liver and kidneys into the active hormone that regulates calcium and phosphate absorption and bone mineralisation.
Its established role is skeletal. Deficiency causes rickets in children and osteomalacia in adults, and correcting a deficiency clearly matters for bone health, particularly alongside adequate calcium and resistance training.
Latitude makes it a genuine public health issue in the UK. Between roughly October and March the sun is too low for the skin to make meaningful amounts, which is why UK guidance suggests considering a daily supplement through the autumn and winter, and year round for people with little sun exposure or darker skin.
The wider claims are where the evidence thins out. Observational studies link low vitamin D to almost everything, and large randomised trials of supplementation have mostly failed to show the benefits those associations implied for cancer, cardiovascular disease and, in most cases, athletic performance. Low vitamin D looks partly like a marker of being unwell or inactive rather than only a cause.
For performance specifically, correcting a deficiency can help; supplementing someone already replete does not, which is the same plateau shape seen across micronutrients.
More is not better, and it is one of the few vitamins where excess is genuinely toxic, causing high calcium levels through sustained very high doses. D3 raises blood levels more effectively than D2.
In practice: consider a modest daily supplement in UK winters, test rather than guess if you suspect a real deficiency, and be sceptical of anything promising benefits beyond bone and muscle.
Where your own result would sit
Most people reading this page are holding a blood-test report with this marker on it, and the question they arrived with is whether their number is normal. A reference interval says where most results from a healthy population fall; a guideline threshold is where a clinical body draws a line for a decision. Neither is a target, and a result on the wrong side of one is not a diagnosis: the same number can be unremarkable in one person and worth looking into in another, which is why the clinician who ordered the test reads it against your history rather than against a list.
Vitamin D has no published interval on this page yet. We hold one, and the dashboard draws it beside a reader’s own result, but it has not been read back against the laboratory’s own page since it was collected. A figure printed beside somebody’s blood result has to have been seen at its source, not remembered, so this one waits.
On your own blood test?
Vitamin D is one of the markers FitTools can read from an uploaded blood-test report. You confirm every value before anything is saved, and your results are explained together in one place.