For years, a hopeful story has circulated in men's health circles: low testosterone is tied to diabetes, so raising testosterone should help fix the blood sugar. A new 4-year follow-up of the landmark T4DM trial, presented at ENDO 2026 in Chicago, puts a firm boundary on that idea.
The headline finding: testosterone's effect on blood sugar is real but front-loaded and fades, while its effects on libido and body composition stick around. Translation — testosterone is a useful adjunct, not a diabetes drug.
Key Takeaways
- 4-year follow-up of 121 men from the original 1,007-man T4DM trial, presented at ENDO 2026
- Most blood-sugar improvement happened in the first 2 years and diminished by year 4
- Glucose control still beat placebo at year 4, but the gap narrowed
- Sexual desire and body-composition gains (less fat, more muscle) held through year 4
- Overall quality of life did not clearly improve versus placebo at any point
- No new safety concerns over four years
- Lead investigator: testosterone is not a substitute for lifestyle change or standard diabetes care
What the T4DM Trial Was
T4DM — Testosterone for the Prevention of Type 2 Diabetes Mellitus — is one of the most rigorous tests of testosterone in metabolic disease ever run. The original randomized, double-blind, placebo-controlled trial enrolled 1,007 men aged 50-74 with a waist circumference of at least 95 cm and either impaired glucose tolerance or newly diagnosed type 2 diabetes.
Every participant went into a structured lifestyle program — diet and exercise — and was then randomized to injectable testosterone or placebo for two years. The original result, published in 2021, was striking: adding testosterone to the lifestyle program cut progression to type 2 diabetes by roughly 40% versus lifestyle plus placebo.
That number launched a thousand marketing claims. The 4-year extension is the reality check.
The Original Trial at a Glance
| Parameter |
Detail |
| Trial |
T4DM (Testosterone for the Prevention of Type 2 Diabetes) |
| Original enrollment |
1,007 men, age 50-74 |
| Inclusion |
Waist ≥95 cm + impaired glucose tolerance or early T2D |
| Intervention |
Lifestyle program + injectable testosterone vs. lifestyle + placebo |
| Original duration |
2 years |
| Original headline |
~40% lower progression to T2D with testosterone added |
| Lead investigator |
Gary Wittert, Adelaide University / Royal Adelaide Hospital |
What the 4-Year Follow-Up Added
The ENDO 2026 presentation tracked a subgroup of 121 men who chose to continue blinded treatment — testosterone or placebo — for two additional years. The key difference: they did this without ongoing enrollment in the structured lifestyle program.
That design quirk is exactly what makes the follow-up useful. It isolates what testosterone does on its own once the diet-and-exercise scaffolding comes down.
Here's how the outcomes split.
| Outcome |
First 2 years |
By year 4 |
| Blood sugar control |
Strong improvement |
Diminished, but still better than placebo |
| Sexual desire |
Improved |
Maintained |
| Body composition (fat down, muscle up) |
Improved |
Maintained |
| Overall quality of life |
No clear benefit |
No clear benefit |
| Safety |
No concerns |
No new concerns |
The pattern is clear. The metabolic win regressed once the lifestyle program ended, even though men kept taking testosterone. The libido and physique wins held, because those respond more directly to maintained testosterone levels.
Why the Glucose Benefit Faded
This is the part worth understanding, because it changes how you should think about TRT and metabolism.
The first two years stacked two things on top of each other: a structured lifestyle program and testosterone's effect on body composition. Less visceral fat and more muscle improve insulin sensitivity directly — that's most of where the early glucose benefit came from. When the lifestyle program stopped, the behavioral half of that engine switched off, and glucose drifted back toward the placebo group.
Testosterone kept doing its part — the men held onto their leaner body composition — but body composition alone wasn't enough to keep glucose control improving without the diet and activity changes driving it. This lines up with the 2026 NHANES analysis showing that metabolic health is the dominant driver of testosterone status in the first place. The two stories reinforce each other: metabolism and testosterone are tightly linked, but metabolism is upstream.
Lead investigator Gary Wittert put it bluntly: testosterone treatment alone is not a replacement for lifestyle intervention, weight management, or standard diabetes prevention in older men with central obesity and prediabetes or early type 2 diabetes.
What This Means If You Have Prediabetes or Early Diabetes
If you're a man in your 50s or 60s with a thick waistline, borderline glucose, and low-T symptoms, this study has direct implications.
Do not treat TRT as a blood-sugar drug. The durable benefits here were libido and body composition — both legitimate quality-of-life wins, but not the same as glycemic control. Your diabetes risk is managed by the same tools that always managed it: sustained weight loss, resistance training, walking, sleep, and, where indicated, a GLP-1 or metformin. See how a GLP-1 stack interacts with testosterone if weight is your primary lever.
If you also genuinely have testosterone deficiency, TRT can earn its place as an adjunct. Improved body composition makes the metabolic work easier — it's easier to keep muscle and lose fat when testosterone is in range — and the libido and energy gains help adherence. But it's a support beam, not the foundation.
Get the diagnosis right before the prescription. A recent ENDO 2026 study found only 12% of TRT prescriptions had a guideline-concordant workup. Men with metabolic disease are exactly the population most likely to get prescribed TRT on a single symptom checklist when the real driver is fixable metabolic dysfunction. Insist on morning total and free testosterone, a repeat confirmatory draw, LH/FSH, and a full safety panel before committing. A vetted clinic does this by default — see how to choose a TRT clinic for the criteria that separate real medicine from a prescription mill.
How This Fits the 2026 Testosterone Picture
This study lands in a busy year for testosterone science. The TRAVERSE cardiovascular safety data cleared TRT of the major heart-risk fears that dogged it for a decade. The FDA moved to remove the boxed cardiovascular warning and is exploring a label expansion for low libido in idiopathic hypogonadism. Real-world cohorts of thousands of men keep confirming the safety profile when TRT is properly managed.
The T4DM 4-year data fits that maturing picture without contradicting it. Testosterone is safe and it does real things. But it does specific things — sexual function, body composition, vitality in deficient men — and it doesn't do everything. It is not a metabolic cure, and the trial that produced the most optimistic diabetes-prevention number is now the trial telling you to keep your expectations grounded.
The Limits of This Follow-Up
A few caveats keep this honest. The 4-year subgroup was small — 121 men — and self-selected, since participants chose to continue. Smaller samples widen the error bars and self-selection can skew results in either direction. The follow-up also dropped the lifestyle program by design, which is the point but also means it doesn't tell you what happens to men who keep both testosterone and the diet-and-exercise habits going for four years. That combined-maintenance scenario is arguably the most relevant one for real patients, and it wasn't tested here.
None of that undermines the core message. If anything, the small confirmatory signal that glucose still beat placebo at year four — despite no lifestyle program — suggests testosterone retains a modest metabolic role. It's just nowhere near the headline most marketing implies.
Bottom Line
The 4-year T4DM follow-up is a useful dose of realism. Testosterone's blood-sugar benefit in at-risk older men is front-loaded and fades once you stop doing the metabolic work; its benefits for libido and body composition hold. The practical lesson is the same one good clinicians have repeated for years: fix the metabolism with metabolic tools, and use testosterone for what it actually treats.
If you have prediabetes or early diabetes plus genuine testosterone deficiency, TRT can be a legitimate adjunct that makes the hard work easier — but only after a proper diagnosis. Manage the diabetes like diabetes, and treat the low T like low T.
Related Reading
References
- Wittert G, et al. Four-year follow-up of testosterone treatment effects on glycemic control, sexual function, and body composition in men at risk of type 2 diabetes (T4DM extension). Presented at ENDO 2026, Chicago, IL, June 13-16, 2026.
- 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.
- Lincoff AM, et al. Cardiovascular safety of testosterone-replacement therapy (TRAVERSE). N Engl J Med. 2023;389(2):107-117.
- American Urological Association. Evaluation and Management of Testosterone Deficiency Guideline. 2018, updated 2024.