The panel / The causes nobody else checks

Vitamin D, B12

Both of these get oversold in men's health marketing, and both still belong on the panel. The honest position is that deficiency is common and worth correcting, while the promise that correcting it will transform how you feel is considerably thinner than the supplement aisle suggests.

What it measures

25-hydroxyvitamin D is the storage form and the standard measure of vitamin D status. Total B12 is the first-line test, though it is a blunt instrument, because only around a fifth of circulating B12 is in the metabolically active fraction.

What each test shows

25-hydroxyvitamin DReflects vitamin D stores

Total serum B12The screening test, with a borderline zone

Methylmalonic acidConfirms genuine deficiency when B12 is borderline

Total B12 has false positive and false negative rates reported as high as 50 percent. That is why a borderline result gets a confirmatory test rather than a supplement.

Oral B12 is as effective as intramuscular for most people, which is worth knowing if you have been offered a course of injections. Injections are preferred for severe deficiency or where there are neurological symptoms.

Why it is on the panel

B12 deficiency produces fatigue and neurological symptoms, and is caused by two extremely common medications: metformin and long-term acid blockers. Vitamin D deficiency is associated with low mood and low testosterone in observational data. Both are cheap to test and cheap to fix. Note that routine screening in asymptomatic adults is not recommended, which is a different question from testing a man who has arrived with symptoms.

How it is measured

SpecimenSerum

TimingWith the rest of the panel

FastingNot required

If B12 is borderlineMethylmalonic acid follows

TurnaroundTypically 2 to 4 days

Repletion routeOral is non-inferior to injection for most men

Interpretation

Reading the result

For vitamin D there is genuinely no agreed cutoff, and bodies disagree openly. We would rather show you the disagreement than pretend to a precision that does not exist.

Bands used in practice

Vitamin D under 12 ng/mLDeficient by the National Academy of Medicine definition.

Vitamin D around 20 ng/mLConsidered adequate for most people by that same standard.

Vitamin D 30 ng/mLThe Endocrine Society target, which is a higher bar than others use.

B12 under 180 pg/mLDiagnostic of deficiency.

B12 180 to 350 pg/mLBorderline. Methylmalonic acid decides.

Preserve your own laboratory's reference range, particularly for B12, where published cutoffs vary between 150 and 200 pg/mL depending on the source.

What distorts these results

7 things that move it

Metformin, which reduces B12 absorption in the ileum. In trial data, low or borderline B12 occurred in 19.1 percent of metformin users against 9.5 percent on placebo.

Proton pump inhibitors and H2 blockers, with risk rising after about two years of use.

Autoimmune gastritis, where anti-intrinsic factor antibodies can produce a falsely normal B12 result.

Kidney disease, dehydration and thyroid disease, all of which falsely raise methylmalonic acid.

A vegan or strict vegetarian diet, and alcohol use disorder.

Age. B12 deficiency affects around 6 percent at 60 and around 20 percent by 85.

Bariatric surgery and terminal ileal disease, both of which impair absorption.

What happens next

A borderline B12 gets methylmalonic acid before anything is prescribed, because supplementing a man who is not deficient will not help his fatigue.

Confirmed B12 deficiency without an obvious dietary or drug cause prompts a look for autoimmune gastritis.

Vitamin D deficiency is corrected, with an honest expectation set about what that is likely to change.

If neurological symptoms are present, treatment is not delayed, because early damage is reversible and established damage is not.

Straight answers

Probably not on the evidence, though you will see this claimed constantly. The positive relationship between vitamin D and testosterone comes from observational studies, which cannot separate cause from association, and it has not been confirmed in good randomised trials. Correct a deficiency because deficiency is worth correcting, not because it is a testosterone strategy.