Vitamin D (25-OH-D): why your "optimal" target is a convention, not a trial result

Two things decide whether your vitamin D result means anything, and neither is the number itself. The first is which unit your lab printed it in. The second is that the "optimal" band you are measuring yourself against was never the output of a trial — and in 2024 the society that wrote the most-quoted threshold stopped endorsing it.

Two units, one blood sample

Vitamin D status is reported as 25-hydroxyvitamin D, written 25(OH)D, 25-OH-D or sometimes calcidiol. American laboratories report it in nanograms per millilitre (ng/mL). Most of the rest of the world — the UK, Europe, Australia, Canada — reports the same molecule in nanomoles per litre (nmol/L). The two differ by a factor of almost exactly 2.5: multiply ng/mL by 2.496 to get nmol/L, or multiply nmol/L by 0.4006 to go the other way. Micrograms per litre (µg/L) is numerically identical to ng/mL and needs no conversion at all.

Established clinical use Guideline documents quote both units side by side precisely because the confusion is so common — the Institute of Medicine's threshold is written as 20 ng/mL (50 nmol/L). ¹ Notice what that means in practice. A UK reader whose report says 75 nmol/L has 30 ng/mL. Read against a community target of "50 to 80", that 75 looks comfortably mid-range when it is in fact sitting exactly on the lowest line any guideline body has ever drawn. It is the easiest misreading of a vitamin D result there is, and it runs in one direction: people reading nmol/L conclude they are fine against a target that was written for ng/mL.

The reverse error is rarer and worse. A US reader who meets a European figure of 50 nmol/L and treats it as 50 ng/mL has silently multiplied their goal by two and a half. Before comparing your number to anything — a forum post, a guideline, this page — check the unit printed on your own report, and if a tool converted it for you, check it converted in the direction you assumed.

25-OH-D is the test. 1,25-dihydroxy is not.

Vitamin D from sunlight or a supplement is hydroxylated in the liver to 25-hydroxyvitamin D, which circulates bound to vitamin D binding protein with a half-life of two to three weeks. That long half-life and large circulating pool are exactly why 25-OH-D is the storage marker and the right thing to measure: it integrates weeks of intake and sun exposure rather than reflecting yesterday. A second hydroxylation, mostly renal, produces 1,25-dihydroxyvitamin D — calcitriol, the hormonally active form.

Established clinical use 1,25-dihydroxyvitamin D is not a test of vitamin D status, and ordering it as one is a recognised laboratory-utilisation error. Its half-life is hours, its concentration is roughly a thousandth of 25-OH-D's, and it is held tightly by parathyroid hormone — so as stores fall, PTH rises and drives 1,25 up, not down. Someone with genuine deficiency can therefore return a perfectly normal or even high 1,25. Laboratory-medicine guidance restricts the test to suspected disorders of vitamin D metabolism: hypercalcaemia of unclear cause, granulomatous disease such as sarcoidosis, chronic kidney disease, inherited rickets. ²

So if a panel you paid for returned "1,25-dihydroxy vitamin D" and no 25-hydroxy, you did not get a vitamin D status result, whatever the number said. Ask for the 25-hydroxy. If it returned both and nobody is investigating your calcium metabolism, treat the 1,25 line as noise on the report rather than something to trend.

The assay changes the answer

There are two families of method. Automated immunoassays — Roche, Abbott, DiaSorin, Siemens, Beckman — are what most high-volume laboratories run, because they are fast and cheap. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) physically separates the metabolites before measuring them, and is the reference approach. Both report a figure labelled "total 25-OH-D". They do not always agree.

Established clinical use The disagreement is not academic. In one large German cohort, women had been measured on a DiaSorin Liaison immunoassay and men on an IDS-iSYS; applying a 30 nmol/L deficiency cut-off classified 48.3% of women and 12.1% of men as deficient. Re-measuring a subset by LC-MS/MS and standardising both platforms collapsed that gap to 15.7% and 14.3% — the apparent sex difference was almost entirely the assay. ³ Standardisation programmes such as DEQAS have narrowed the spread since, but reviews of current assay performance still find meaningful between-method variation. ⁴

Two specific interferences are worth knowing about. The C-3 epimer of 25-OH-D3 is a distinct molecule that some immunoassays cross-react with and that some LC-MS/MS methods fail to separate chromatographically; either way the reported total is inflated. ⁵ And if you take ergocalciferol (D2) rather than cholecalciferol (D3), platforms differ in how completely they recover the D2 form, so a disappointing result may be the method rather than you. The practical consequence is simple. A few ng/mL of movement between two laboratories may be entirely analytical. Track on one platform, and when the platform changes, treat the series as broken rather than as a trend.

Where 30, 50 and 80 came from — and why nobody agrees

The number you have been told to aim for has a traceable origin. It is worth following it back, because the history is the answer.

Established clinical use In 2011 the Endocrine Society published a clinical practice guideline defining deficiency as below 20 ng/mL, insufficiency as 21 to 29 ng/mL, and sufficiency as 30 ng/mL or above. ⁶ In the same year the Institute of Medicine, reviewing largely the same literature, concluded that 20 ng/mL (50 nmol/L) covers the requirements of at least 97.5% of the population, set the recommended dietary allowance at 600 IU daily for most adults and 800 IU above age 70, and flagged risk of harm above 125 nmol/L. ¹ Two expert panels, one body of evidence, thresholds 10 ng/mL apart.

They were not being careless. They were answering different questions. The IOM's remit was a population nutrient requirement — what intake keeps nearly everybody above the level needed for bone health. The Endocrine Society was writing for a clinician facing one patient who might be at risk, and set the bar where it judged that individual was unambiguously covered. A population-coverage question and an individual-sufficiency question have different correct answers even from identical data. Most pages that quote "30" never mention that "20" exists, or why.

Established clinical use The part almost nobody repeats is what happened in 2024. The Endocrine Society issued a new guideline and withdrew its own thresholds. It no longer endorses the 2011 definitions of sufficiency (at least 30 ng/mL) or insufficiency (20 to 30 ng/mL), on the stated grounds that trial evidence does not identify a 25-OH-D concentration predicting net benefit from supplementation in generally healthy people. The same guideline recommends against routine 25-OH-D screening in healthy adults, and suggests supplementation without testing only for defined groups: children and adolescents, adults over 75, pregnancy, and high-risk prediabetes. ⁷ If your target came from that society, the society has moved.

Animal-only or theoretical The 50 to 80 ng/mL band that circulates in training and hormone-optimisation communities has no guideline body behind it at all. It is an extrapolation: observational cohorts consistently show better outcomes in people with higher 25-OH-D, and the band was drawn where those associations look strongest. TherapyLog displays it because you will meet it everywhere and it helps to see where you sit relative to it. It is non-diagnostic. It was not validated against outcomes in any trial, and being outside it is not a finding. Your own laboratory's reference interval, printed on your own report and matched to the assay your own laboratory ran, is the range that governs.

What the big trials found, and what they did not

Established clinical use Between 2019 and 2023 the question finally got the trials it deserved. VITAL randomised 25,871 US adults to 2000 IU of vitamin D3 daily or placebo and found no reduction in invasive cancer or major cardiovascular events. ⁸ Its fracture analysis found no reduction in total, non-vertebral or hip fractures. ⁹ D-Health randomised 21,315 Australians aged 60 and over to monthly vitamin D3 or placebo for five years and found no reduction in all-cause mortality. ¹⁰ Its fracture outcome was null as well. ¹¹ These are large, long, well-conducted trials, and they repeatedly failed to reproduce what the observational literature had predicted.

It matters what those results do and do not license you to conclude. Mean baseline 25-OH-D in VITAL was 30.8 ng/mL, and only 12.7% of participants were below 20 ng/mL — the population was largely replete before randomisation. ⁸ The fair reading is therefore not "vitamin D does nothing". It is that pushing an already-adequate person higher does not buy the outcomes the associations promised. Correcting genuine deficiency is a separate question, and the evidence that vitamin D prevents rickets and osteomalacia was never in dispute.

Animal-only or theoretical For this audience the specific belief worth addressing is that raising vitamin D raises testosterone. The observational association is real and reproducible — men with lower 25-OH-D do have lower testosterone on average. The randomised evidence does not support causation. In a placebo-controlled trial in middle-aged men with normal baseline testosterone, vitamin D had no effect on total testosterone, though it did raise oestradiol and lower SHBG. ¹² A second trial restricted to men with low baseline testosterone found no significant effect on total testosterone, free testosterone or free androgen index. ¹³ Correcting a genuinely low vitamin D is defensible on its own terms. Expecting it to move your testosterone is not, and it is not a substitute for investigating why your testosterone is low.

Established clinical use Vitamin D is treated as harmless in most of the communities that push the high targets, and it is not. Toxicity is real, mediated by hypercalcaemia, and typically appears at 25-OH-D above 150 ng/mL (375 nmol/L) alongside suppressed parathyroid hormone. It presents as nausea, vomiting, polyuria, thirst, dehydration, confusion and weakness, and can progress to nephrocalcinosis, acute kidney injury and arrhythmia. ¹⁴ Published cases are overwhelmingly the result of sustained very large intakes over months, sometimes compounded by mislabelled products, rather than ordinary supplementation. The IOM set a tolerable upper intake level of 4000 IU daily for adults, and the gap between that and the intakes in the toxicity case series is wide. ¹ But wide is not infinite, and fat-soluble means it accumulates.

None of that is a reason to panic about an ordinary dose, and all of it is a reason that a sustained high intake belongs on the record of the clinician who prescribes for you — with the result, the unit and the assay in front of them. Whether it needs changing is their call, not this page’s.

So the sequence for a result you are unsure about is this. Confirm the unit. Confirm you are looking at 25-hydroxy and not 1,25. Compare it against the interval your own laboratory printed, on the assay your own laboratory ran, rather than against a band from a forum. If it is genuinely low, genuinely high, or has moved a long way without you changing anything, that is a conversation with the clinician who prescribes for you — bring the report, the units and the method, and let them decide what, if anything, should change. A number on a screen is not a treatment decision, and neither is this page.

What the app records for this marker

Compounds whose monitoring panel includes this marker

Generated by matching this marker's own aliases against the monitoring note on every compound in the app's reference, so the list is the app's, not an author's.

• LL-37 CBC (immune cells), Vitamin D levels (essential), CRP. Monitor injection sites for local reactions.
• Sigumir Joint pain VAS scale, functional movement assessment, Vitamin D levels (critical for cartilage health).

Questions people actually ask

My result says 75 nmol/L. Is that good?

75 nmol/L is 30 ng/mL. That is exactly the old Endocrine Society sufficiency line and well above the Institute of Medicine's 20 ng/mL (50 nmol/L). It is not, however, anywhere near the 50 to 80 ng/mL band that circulates in training communities — that band would be 125 to 200 nmol/L.

This is the single most common misreading of a vitamin D result. If you are in the UK, Europe, Australia or Canada, your result is almost certainly in nmol/L, and the American targets you are reading are in ng/mL. The two are 2.5x apart. Compare like with like, and compare against the interval your own laboratory printed.

Is 30 ng/mL enough, or do I need to be at 50?

There is no settled answer, and anyone who gives you one confidently is picking a side without telling you. The Endocrine Society said 30 in 2011. The Institute of Medicine said 20 in the same year, from largely the same evidence, because it was answering a population-requirement question rather than an individual-sufficiency one.

In 2024 the Endocrine Society withdrew its own thresholds, stating that the trial evidence does not identify a 25-OH-D concentration that predicts net benefit from supplementation in generally healthy people. The 50 figure has no guideline body behind it at all. Your laboratory's own reference interval is the range that applies to your result.

Does vitamin D raise testosterone?

The observational association is real: men with lower 25-OH-D have lower testosterone on average. The randomised evidence does not support the causal step. A placebo-controlled trial in men with normal baseline testosterone found no effect on total testosterone, and a companion trial restricted to men with low baseline testosterone found no effect on total testosterone, free testosterone or free androgen index.

Correcting a genuinely low vitamin D is worth doing for its own reasons. Treating it as a testosterone intervention is not supported, and it should not delay looking into why your testosterone is low.

Why did my vitamin D drop when I changed labs?

Very possibly because the assay changed. Immunoassays and LC-MS/MS both report a number labelled 'total 25-OH-D' and they do not always agree; in one cohort, standardising two immunoassay platforms against mass spectrometry eliminated what had looked like a large sex difference in deficiency prevalence.

Interferences differ by platform too — the C-3 epimer inflates some results, and recovery of the D2 form varies if you take ergocalciferol. Track on one platform where you can, and when the method changes, treat the run as a new series rather than a fall.

Should I get the 1,25-dihydroxy vitamin D test as well?

Not for assessing your vitamin D status. 1,25-dihydroxyvitamin D has a half-life of hours and is tightly controlled by parathyroid hormone, so it commonly rises as stores fall. Someone genuinely deficient can return a normal or high 1,25.

It is a legitimate test for a narrow set of problems — unexplained hypercalcaemia, granulomatous disease, chronic kidney disease, inherited disorders of vitamin D metabolism. If none of those is being investigated and you have both numbers on a panel, the 25-hydroxy is the one that carries information.

Can you actually take too much vitamin D?

Yes. Toxicity is mediated by hypercalcaemia and typically appears with 25-OH-D above 150 ng/mL (375 nmol/L) and suppressed parathyroid hormone, presenting as nausea, vomiting, excessive urination, thirst, dehydration, confusion and weakness, and capable of progressing to kidney injury and arrhythmia.

Published cases are dominated by sustained very large intakes over months, sometimes made worse by mislabelled products — not by routine supplementation. The IOM's tolerable upper intake level for adults is 4000 IU a day. Vitamin D is fat-soluble and accumulates, so if you are taking a high intake, the person who prescribes for you should know about it and should be the one deciding whether it continues.

Sources

Every claim above that is not a definition is either labelled by evidence tier or carries a numbered reference to one of these.

1. Ross AC, Manson JE, Abrams SA, et al. The 2011 Report on Dietary Reference Intakes for Calcium and Vitamin D from the Institute of Medicine: What Clinicians Need to Know. J Clin Endocrinol Metab 2011;96(1):53-58 The IOM committee's own summary: 20 ng/mL (50 nmol/L) covers at least 97.5% of the population, with the RDA set at 600 IU (800 IU above 70), an upper intake level of 4000 IU/day, and risk of harm flagged above 125 nmol/L.
2. Association for Diagnostics & Laboratory Medicine (ADLM), Optimal Testing: Guide to Lab Test Utilization — Vitamin D Laboratory-medicine guidance that 25-hydroxyvitamin D is the test of status and that 1,25-dihydroxyvitamin D is reserved for suspected disorders of vitamin D metabolism.
3. Schöttker B, Jansen EHJM, Haug U, Schomburg L, Köhrle J, Brenner H. Standardization of Misleading Immunoassay Based 25-Hydroxyvitamin D Levels with Liquid Chromatography Tandem-Mass Spectrometry in a Large Cohort Study. PLoS ONE 2012;7(11):e48774 Shows an apparent 48.3% versus 12.1% sex difference in deficiency prevalence collapsing to 15.7% versus 14.3% once two immunoassay platforms were standardised against LC-MS/MS.
4. Analytical variation concerning total 25-hydroxyvitamin D measurement, where are we now? A DEQAS review of current assay performance. J Steroid Biochem Mol Biol (2023) External quality assessment review confirming that between-method variation in total 25-OH-D persists despite standardisation efforts.
5. 25-Hydroxyvitamin D assays: Potential interference from other circulating vitamin D metabolites. J Steroid Biochem Mol Biol Documents interference in 25-OH-D measurement from related metabolites, including the C-3 epimer, and how it differs by platform.
6. Holick MF, et al. Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab 2011;96(7):1911-1930 The origin of the widely quoted thresholds: deficiency below 20 ng/mL, insufficiency 21-29 ng/mL, sufficiency 30 ng/mL or above.
7. Demay MB, Pittas AG, Bikle DD, et al. Vitamin D for the Prevention of Disease: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab 2024;109(8):1907-1947 The 2024 reversal: the Society no longer endorses its 2011 sufficiency and insufficiency definitions, recommends against routine 25(OH)D screening in healthy adults, and limits empiric supplementation to defined groups.
8. Manson JE, Cook NR, Lee IM, et al. Vitamin D Supplements and Prevention of Cancer and Cardiovascular Disease. N Engl J Med 2019;380(1):33-44 VITAL: 25,871 adults, 2000 IU/day, no reduction in invasive cancer or cardiovascular events; mean baseline 25-OH-D 30.8 ng/mL with 12.7% below 20 ng/mL.
9. LeBoff MS, et al. Supplemental Vitamin D and Incident Fractures in Midlife and Older Adults. N Engl J Med 2022;387(4):299-309 VITAL's fracture endpoint: no lower risk of total, non-vertebral or hip fracture with vitamin D3 versus placebo.
10. Neale RE, et al. The D-Health Trial: a randomised controlled trial of the effect of vitamin D on mortality. Lancet Diabetes Endocrinol 2022;10(2):120-128 D-Health: 21,315 Australians aged 60+, monthly vitamin D3 for five years, no reduction in all-cause mortality.
11. The effect of monthly vitamin D supplementation on fractures: a tertiary outcome from the population-based, double-blind, randomised, placebo-controlled D-Health trial. Lancet Diabetes Endocrinol 2023 D-Health's fracture outcome, also null, including non-vertebral and major osteoporotic fractures.
12. Lerchbaum E, et al. Vitamin D and Testosterone in Healthy Men: A Randomized Controlled Trial. J Clin Endocrinol Metab 2017;102(11):4292 Placebo-controlled trial in men with normal baseline testosterone: no effect on total testosterone, with an increase in oestradiol and a decrease in SHBG.
13. Lerchbaum E, et al. Effects of vitamin D supplementation on androgens in men with low testosterone levels: a randomized controlled trial. Eur J Nutr (2019) Companion trial in men with low baseline testosterone: no significant effect on total testosterone, free testosterone or free androgen index.
14. Marcinowska-Suchowierska E, Kupisz-Urbańska M, Łukaszkiewicz J, Płudowski P, Jones G. Vitamin D Toxicity–A Clinical Perspective. Front Endocrinol 2018;9:550 Clinical review of vitamin D toxicity: hypercalcaemia, typical 25-OH-D above 150 ng/mL (375 nmol/L), symptoms and the sustained very high intakes that cause it.