Blog · Lifestyle & Wellness

What Causes Low Testosterone — and Can You Reverse It?

By Vitae Team · 2026-10-10

Low testosterone has become one of the most heavily marketed diagnoses in men's health, and one of the least precisely defined. Here is what sets the threshold, what the evidence says causes it, how much of it reverses, and where the treatment claims outrun the data.

TL;DR

There is no single number that defines it. Across nine major guidelines, the cutoff for total testosterone ranges from 8 to 12 nmol/L — a 50% spread in who counts as deficient.
The UK threshold moved recently. BSSM set treatment at below 8 nmol/L in 2017 and below 12 nmol/L in 2023.
Symptoms are not a diagnosis. In a 2025 UK survey, 49% of men screened positive on a symptom questionnaire while about 5% had a formal diagnosis.
The commonest cause is weight. Testosterone rises in proportion to weight lost, by roughly 0.6% per kilogram.
Reversal takes more than a diet. One year after sleeve gastrectomy, biochemical remission reached 89% in men losing over 21.8% of body weight, against 47% below it.
Supplements do not deliver. A 2026 review of fenugreek found a small effect on total testosterone, none on free testosterone, and rated both very low certainty.

There Is No Single Number

Most men meet the diagnosis through a blood test, which makes the threshold the most consequential fact about it. A 2026 commentary in the International Journal of Impotence Research, reviewing nine major specialist guidelines, found total testosterone cutoffs ranging from 8 nmol/L to 12 nmol/L. The authors note that this spread cannot be explained by assay or population differences alone, and that it creates a grey zone affecting a substantial share of the men being assessed.

The UK sits at the permissive end of that range, and arrived there recently. The British Society for Sexual Medicine set the treatment level at a total testosterone below 8 nmol/L in its 2017 guideline, with 8 to 12 as a zone for a trial of therapy. Its 2023 update moved the treatment level to below 12 nmol/L, with a trial zone of 8 to 14, and support for treating below 14 in men with prediabetes. The condition did not change in those six years. The number did.

One detail from that guidance is worth carrying into any private clinic. BSSM advises using published treatment action levels rather than laboratory reference ranges, which means the "normal range" printed beside your result is not the thing that decides anything. It also requires two morning samples, taken between 8 and 11am, before any conclusion is drawn.

Symptoms sit in a similarly uncertain relationship to the biochemistry. A 2025 survey of 973 UK men found 49% screening positive on a symptom questionnaire while around 5% reported a formal diagnosis. That gap is not a measure of undiagnosed disease: it was a screening instrument given to a paid online panel, not a representative sample with confirmatory blood tests. For comparison, when European researchers applied a stricter definition requiring both symptoms and low testosterone to 2,966 men aged 40 to 79, 2.1% met it.

What Causes It

Testosterone falls with age, but age is rarely the whole story in men under seventy. The dominant reversible cause is body weight, through what researchers call functional or secondary hypogonadism: excess adipose tissue alters the hormonal signalling that drives testosterone production, and the testes are not themselves damaged. Type 2 diabetes, metabolic syndrome and chronic illness travel with it.

Several medications suppress testosterone directly, including opioids, oral glucocorticoids and antipsychotics, and androgen deprivation therapy does so by design. Genuine primary hypogonadism, where the testes have failed, is a different condition requiring different management, and very low readings with low gonadotropins warrant pituitary imaging rather than a prescription.

The claim that testosterone is falling across the population appears to be real, and is not explained by obesity. A study of 991 American men found mean testosterone fell 207 ng/dL between 1982 and 2002, with a secular component of around 8.9 ng/dL per year against 3.9 from ageing alone. Weight gain explained 16% of the variance, and men who stayed weight-stable still declined by about 117 ng/dL. The authors explicitly excluded rising obesity as a sufficient explanation, and said they had not identified the cause. Anyone who tells you they know why is ahead of the evidence.

Can You Reverse It?

In the obesity-driven form, substantially yes, and the effect scales with weight lost. A meta-analysis of 24 studies found total testosterone rose by 2.87 nmol/L (95% CI 1.68–4.07) after a low-calorie diet and 8.73 nmol/L (95% CI 6.51–10.95) after bariatric surgery, with the degree of weight loss the strongest predictor. A 2024 prospective study put the relationship at 0.6% per kilogram lost (95% CI 0.2–1.0%), with no difference between diet and surgery once weight loss was accounted for.

The size of the required change is the sobering part. In 230 men followed for a year after sleeve gastrectomy, biochemical remission reached 88.9% among those losing between 21.8% and 36% of their body weight, against 47.1% below that threshold. The authors describe the threshold as exploratory and in need of prospective validation, but the direction is clear: around a fifth of body weight, not a few kilograms.

There is also a tension in the guidance that readers should know about. BSSM's position is that current evidence does not support lifestyle intervention alone as first-line therapy for symptomatic men, and it recommends combining weight loss with testosterone therapy, citing European research in which weight loss alone did not improve sexual desire or erectile function. So the UK guideline holds that the cause is largely reversible and that reversing it may not resolve the symptoms. Both things can be true, and it leaves a real decision rather than an obvious one.

What About Treatment?

The largest safety trial is reassuring on its primary question and more complicated underneath. TRAVERSE randomised 5,204 men with hypogonadism and cardiovascular disease or risk factors to testosterone gel or placebo, with a median follow-up of 32.9 months. Testosterone was non-inferior for the main cardiovascular endpoint of death, heart attack or stroke, with a hazard ratio of 0.96. That is the finding usually reported.

The secondary results are reported far less often. Atrial fibrillation occurred in 3.5% of the testosterone group against 2.4% on placebo (p=0.02), arrhythmias requiring intervention in 5.2% against 3.3% (p=0.001), acute kidney injury in 2.3% against 1.5% (p=0.04), and venous thromboembolism in 1.7% against 1.2%. Fractures were also increased. "Non-inferior on the primary endpoint" and "safe" are not the same statement.

Those risks interact awkwardly with who gets diagnosed. BSSM notes that obese men are more likely to experience adverse effects from testosterone therapy than men of normal weight — meaning the group most likely to be found deficient is also the group most exposed to the treatment's harms.

It is also worth knowing who is arguing for wider testing. A 2025 consensus exercise published in the Journal of Sexual Medicine, which recommended that testosterone therapy should usually be considered once deficiency is diagnosed, states that it was initiated and funded by Besins Healthcare, a testosterone manufacturer. The company commissioned the analysis and the drafting of the manuscript and reviewed the draft, and all authors received honoraria. It surveyed 55 clinicians recruited by snowball sampling over a single round. That does not make its recommendations wrong, but it is not independent evidence that more men need treatment.

Do Testosterone Boosters Work?

Mostly not, and the best recent evidence is unusually clear about it. A 2026 systematic review of fenugreek, among the commonest booster ingredients, pooled six randomised placebo-controlled trials and 385 men for total testosterone and found a small significant effect, a standardised mean difference of 0.25 (95% CI 0.02–0.48). For free testosterone — arguably the more relevant measure — five trials and 334 men gave 0.08, which was not significant. The authors rated both outcomes very low certainty. Of the thirteen trials included, none was at low overall risk of bias, and the review concluded the evidence does not support commercial testosterone-boosting claims.

A broader review of 52 studies covering 27 proposed boosters reached the same general verdict, that most fail to raise total testosterone, while naming a handful of exceptions as possibly effective in specific groups. The useful point is not which ingredient to try. It is that a small shift in a laboratory number is not the same thing as feeling better, having more strength, or improved sexual function — and none of these products has shown the latter.

What This Actually Means

The most useful thing to understand before being tested is that the threshold is a decision, not a discovery. A reading of 10 nmol/L makes you deficient under one national guideline and normal under another, and the reference range on your result sheet is not what guidelines use. That matters most in private and direct-to-consumer settings, where the test, the interpretation and the prescription may all come from the same organisation.

The second is that the commonest cause is also the most addressable, and that the honest version is demanding rather than discouraging. Testosterone tracks weight loss closely, but biochemical remission in the trials clusters around losing a fifth of body weight, which is a different proposition from the usual advice about sleep and zinc.

The third is that treatment deserves the same scepticism in both directions. Testosterone therapy is not the cardiovascular danger it was once thought to be, and it is not free of harm either. The questions worth asking a clinician are which threshold they are using, whether two morning samples were taken, what is driving the low reading, and what happens to the dose if you lose weight.

FAQ

What is a normal testosterone level in the UK?

There is no single answer. UK guidance generally supports treatment in symptomatic men below 12 nmol/L on two morning samples, while other national guidelines set the line as low as 8. The laboratory reference range on your result is not the threshold guidelines use.

Does losing weight raise testosterone?

Yes, roughly in proportion to the weight lost — about 0.6% per kilogram in one prospective study, and an average rise of 2.87 nmol/L after low-calorie dieting across pooled trials.

How much weight loss is needed to fix it?

More than most people expect. After sleeve gastrectomy, biochemical remission was 89% in men losing over about 22% of body weight, against 47% below that.

Do GLP-1 weight-loss drugs raise testosterone?

Possibly, mainly through weight loss, but the evidence is thin. A 2026 systematic review found mostly retrospective studies, rated the certainty for total testosterone as low, and reported no change in the one randomised placebo-controlled trial, which was in healthy men.

Is testosterone therapy safe for the heart?

The largest trial found no increase in heart attack, stroke or cardiovascular death, but did find more atrial fibrillation, arrhythmias needing intervention, acute kidney injury and venous thromboembolism. It needs discussing with a clinician rather than assuming either answer.

The Bottom Line

Low testosterone is a real condition with a contested definition, and the number that triggers a diagnosis depends on which guideline your clinician follows.

Where weight is driving it, the biochemistry improves as weight comes off, though the amount required is substantial. Supplements have not been shown to help, and testosterone therapy carries real if modest risks that the headline safety findings do not capture.

This article is for information only. If you have symptoms you think may relate to low testosterone, speak to your GP, who can arrange appropriate testing and interpretation rather than relying on a single result.

For a broader, structured approach to the habits that support long-term health, explore the Vitae Reset guides and pair your plan with the Reset Companion for day-to-day support.

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