Testosterone and Type 2 Diabetes: What the Trials Actually Found

A man of 54 is diagnosed with type 2 diabetes. Over the following year he reads that low testosterone and diabetes travel together, gets tested, comes back at 9.4 nmol/L, and arrives at his next appointment convinced he has found the root cause.

He is half right, and the half he has wrong will cost him two years if nobody corrects it.

The association between low testosterone and type 2 diabetes is one of the strongest in men's endocrinology. What it means for treatment is a separate question, and two large trials have now answered it in ways that appear to contradict each other.

How closely the two travel

Between a quarter and a half of men with type 2 diabetes have low testosterone. That is not a fringe observation, it is a consistent finding across populations.

Most of these men have what is called functional hypogonadism: testosterone is low, but LH and FSH are normal, and the pituitary and testes are capable of working properly. Nothing is broken. The signal is being suppressed by something else.

Low testosterone also predicts diabetes rather than merely accompanying it. In a cohort of 1,655 Australian men followed for five years, low testosterone added measurable predictive information beyond established risk models.

So the relationship is real, it is bidirectional, and it is worth knowing about. The mistake is what men do with that information.

Why they feed each other

Three mechanisms run in a loop, and each one strengthens the next.

Fat converts testosterone to oestradiol. Visceral fat contains aromatase. More fat means more conversion, less testosterone, and more oestrogen feeding back to suppress the pituitary signal.

Insulin resistance suppresses the signal directly. The hypothalamic-pituitary-testicular axis is sensitive to the metabolic environment, and insulin resistance dampens it.

Low testosterone makes fat easier to store and muscle harder to keep. Muscle is where you dispose of most of your glucose. Less muscle means worse glucose handling, which worsens insulin resistance, which lowers testosterone further.

Round it goes. Why your belly fat will not budge covers the first mechanism in detail, and what causes low testosterone places it in the wider list of causes.

Notice what this loop implies. If fat and insulin resistance are driving the low testosterone, then removing them should raise it. That turns out to be exactly what happens, and it matters for what follows.

The trial that made testosterone look like a diabetes treatment

T4DM was designed to answer a specific question: does testosterone prevent type 2 diabetes in men at high risk?

It randomised 1,007 men aged 50 to 74, all with a waist of 95 cm or more, testosterone at or below 14 nmol/L, and either impaired glucose tolerance or newly diagnosed diabetes. Every man received a structured lifestyle programme. Half also received testosterone injections for two years.

The result was striking. At two years, type 2 diabetes was present in 21 per cent of the placebo group and 12 per cent of the testosterone group — a 40 per cent reduction in relative risk.

The treated men also lost more fat, gained muscle and grip strength, and reported better sexual function.

For a while this looked like the answer. Then a second trial reported.

The trial that pointed the other way

TRAVERSE, the large cardiovascular safety trial, included a prespecified diabetes substudy in hypogonadal men aged 45 to 80.

It found no benefit. Testosterone did not reduce progression from prediabetes to diabetes. It did not produce remission in men who already had diabetes. It did not alter glucose or HbA1c. And the null result held regardless of baseline testosterone, age, race, or existing cardiovascular disease.

Two large randomised trials, opposite conclusions. The difference is not a contradiction, and understanding why explains the whole topic.

T4DM gave every man a lifestyle programme. TRAVERSE did not. The T4DM benefit was mediated largely through fat loss, and it appeared on top of an intervention already producing weight loss.

The populations differed. T4DM selected men by waist circumference and impaired glucose tolerance. TRAVERSE selected them by hypogonadism and cardiovascular risk.

T4DM used injections; TRAVERSE used gel.

The reading that fits both: testosterone amplifies what lifestyle change is already doing, and does very little on its own.

What happened when the lifestyle programme stopped

The most useful finding came in 2026, and it settles the question more cleanly than either original trial.

The T4DM extension followed 121 men who continued blinded treatment for two further years, but without the formal lifestyle programme.

At four years, the testosterone group still had lower glucose. But the investigators were blunt about the pattern: most of the benefit had occurred in the first two years, during the lifestyle programme. And among the men at highest risk, those with a baseline two-hour glucose above 11.1 mmol/L, testosterone did not help at all.

Take away the diet and the exercise, and the drug largely stops working.

What the guidance now says

The American Diabetes Association's 2026 Standards of Care addressed this directly and did not hedge: they do not recommend testosterone therapy for the prevention of type 2 diabetes in men with hypogonadism.

That is worth stating plainly, because a great deal of clinic marketing implies otherwise.

It does not mean testosterone is useless if you have both conditions. It means testosterone is a treatment for hypogonadism, not a treatment for diabetes, and the two decisions should be made separately on their own evidence.

What actually works, in order

The interventions with the strongest evidence are the unglamorous ones, and they work on the loop rather than on one node of it.

Fat loss. This is the lever that moves everything. Men who lose substantial weight see testosterone climb in proportion to the fat lost, with no hormone prescribed, and insulin sensitivity improves at the same time.

Resistance training. Muscle is where you dispose of glucose. Building it improves insulin sensitivity independently of weight loss, and it protects you during any weight loss you do achieve. The free workout generator will build a plan.

Sleep, and sleep apnoea in particular. Both conditions are strongly associated with it, and it is missed constantly. Sleep apnoea and testosterone covers what treating it does and does not do.

GLP-1 medication, where indicated. This is the genuinely new part. These drugs produce weight loss that previously required surgery, and the testosterone rise follows the fat loss. One study found a 10 per cent weight reduction accompanied by a 53 to 77 per cent rise in testosterone. GLP-1 medications and testosterone covers the trade-off, because they take muscle as well.

Notice that every item on this list treats the loop. None of them is a hormone.

Where testosterone therapy does fit

It has a place, and the place is narrower than the marketing suggests.

If you have confirmed, symptomatic hypogonadism, testosterone treats that. Sexual function, energy, muscle mass and bone density are the outcomes with evidence behind them. Improved glucose control may follow, largely through changed body composition, and that is a welcome side effect rather than the reason to prescribe.

If your testosterone is low but you have no symptoms, and your real problem is glycaemic control, treating the number is treating the wrong thing.

If you are hoping testosterone will let you avoid the diet and the training, both trials answer that. It will not.

Two practical points for men with diabetes who do start treatment. HbA1c becomes a less reliable marker, because testosterone extends red cell lifespan and can shift the reading independently of your glucose. And haematocrit needs watching closely. TRT side effects covers the monitoring, and TRT and heart health covers the cardiovascular picture, which matters more in this group than most.

What to ask for

If you have type 2 diabetes and symptoms that could be hormonal, this is a reasonable and specific request.

Two morning testosterone samples on separate days, before eleven, with SHBG and calculated free testosterone. A single afternoon reading proves nothing. The blood test guide covers what to ask for.

LH and FSH. These separate functional hypogonadism, where the cause is metabolic and reversible, from primary testicular failure, which is neither. Most men in this situation fall into the first group, and that is good news.

The rest of the panel. Thyroid function, ferritin, and a review of medications, because several suppress testosterone and are commonly prescribed to men with diabetes.

Then, whatever the number shows, treat the loop. The 30-day reset is a structured way to start, and TRT versus natural optimisation covers the wider decision.

The honest summary

Low testosterone and type 2 diabetes are genuinely linked, they make each other worse, and if you have one it is reasonable to check for the other.

But the evidence does not support treating diabetes with testosterone. T4DM's benefit came alongside a lifestyle programme and faded when the programme stopped. TRAVERSE found nothing. The ADA now recommends against it.

The uncomfortable conclusion is the one men least want: the treatment for both conditions is largely the same, and it is not a prescription. Lose the visceral fat, build muscle, fix your sleep, and both numbers move.

Testosterone therapy makes that work better in men who genuinely need it. It does not replace it.

Common questions

Does low testosterone cause type 2 diabetes?

They drive each other. Low testosterone predicts diabetes risk, and insulin resistance suppresses testosterone. Between a quarter and a half of men with type 2 diabetes have low testosterone, usually functional rather than testicular in origin.

Will TRT improve my blood sugar?

Possibly, mainly through changed body composition. But the TRAVERSE diabetes substudy found no effect on glucose or HbA1c, and the ADA's 2026 guidance does not recommend testosterone for preventing diabetes.

What about the T4DM trial that showed 40 per cent risk reduction?

Real, but every man in it was also on a structured lifestyle programme. When the programme stopped in the extension study, most of the benefit had already occurred. Testosterone amplified lifestyle change rather than replacing it.

Can I reverse low testosterone by treating my diabetes?

Often, when it is functional. Weight loss in particular raises testosterone in proportion to fat lost, without any hormone prescribed. LH and FSH tell you whether your case is likely to respond.

Should I get tested if I have diabetes?

Reasonable if you have suggestive symptoms such as low libido, fatigue or loss of morning erections. Two morning samples with SHBG, LH and FSH, rather than a single reading.

Does testosterone affect my HbA1c reading?

It can. Testosterone extends red cell lifespan, which can shift HbA1c independently of your actual glucose control. Worth knowing if your readings change after starting.

Sources: Wittert G et al., Testosterone treatment to prevent or revert type 2 diabetes in men enrolled in a lifestyle programme (T4DM): a randomised, double-blind, placebo-controlled, 2-year, phase 3b trial, Lancet Diabetes Endocrinol 2021;9(1):32-45 · Wittert G et al., T4DM RunOn extension study, presented ENDO 2026 · Effect of testosterone on progression from prediabetes to diabetes in men with hypogonadism: a substudy of the TRAVERSE randomised clinical trial · American Diabetes Association, Prevention or delay of diabetes and associated comorbidities: Standards of Care in Diabetes 2026, Diabetes Care 2026;49(Suppl 1):S50 · Atlantis E et al., Predictive value of serum testosterone for type 2 diabetes risk assessment in men, BMC Endocr Disord 2016;16:26 · Corona G et al., Body weight loss reverts obesity-associated hypogonadotropic hypogonadism, Eur J Endocrinol 2013;168(6):829-43 · Educational only, not medical advice. Never change diabetes medication without speaking to your doctor.

Keep reading: What causes low testosterone · GLP-1 medications and testosterone · Why your belly fat will not budge · Sleep apnoea and testosterone · Blood test: what to check · Free 2-minute testosterone test