Testosterone and Type 2 Diabetes: The Australian T4DM Trial

Type 2 diabetes and low testosterone often travel together in Australian men, particularly those carrying excess weight around the middle. That overlap raises an obvious question: if you raise a man's testosterone, can you improve his blood sugar - or even prevent diabetes? An Australian-led study set out to answer exactly that. The T4DM trial - short for Testosterone for the Prevention of Type 2 Diabetes Mellitus - is one of the largest and most rigorous investigations of testosterone and metabolic health conducted anywhere in the world. Its results are genuinely interesting, and also widely misread. Here is what the trial showed, what it did not, and what it means for men weighing up their options.

The link between low testosterone and type 2 diabetes

Low testosterone and type 2 diabetes are closely connected, and the relationship runs in both directions. Large population studies of middle-aged and older men have shown that lower testosterone is strongly associated with obesity and poorer metabolic health. The mechanism is well described. Fat tissue - especially the visceral fat carried around the abdomen - contains an enzyme called aromatase, which converts testosterone into oestradiol. The more excess fat a man carries, the more testosterone is converted away, and the lower his circulating levels tend to sit. Obesity and insulin resistance also reduce sex hormone binding globulin (SHBG) and can dampen the hormonal signals the brain sends to the testes.

The result can be a self-reinforcing cycle: excess weight lowers testosterone, and lower testosterone can make it easier to gain fat and lose muscle, which in turn worsens insulin resistance. This kind of low reading is often described as "functional" - driven by body weight and metabolic factors rather than a primary fault in the testes - and it is frequently at least partly reversible with weight loss. That distinction matters for everything that follows.

Inside the T4DM trial

T4DM was a randomised, double-blind, placebo-controlled trial run across multiple Australian centres over two years. It recruited men aged 50 to 74 with a waist circumference of 95 cm or more and a serum testosterone of 14 nmol/L or lower - that is, low or low-normal, not the severely low levels seen in classic hypogonadism - who also had either impaired glucose tolerance ("pre-diabetes") or newly diagnosed type 2 diabetes on an oral glucose tolerance test. In other words, these were overweight men whose testosterone sat towards the lower end, exactly the pattern described above.

Every participant was enrolled in a structured lifestyle programme aimed at weight loss and increased physical activity. On top of that shared foundation, the men were randomly assigned to receive either testosterone - as a long-acting testosterone undecanoate injection - or a matching placebo injection for two years. Because neither the men nor their doctors knew who was receiving which, the design gives a reliable read on what the testosterone itself added over and above the lifestyle programme.

What the trial found

The headline result was a real one. At the end of two years, type 2 diabetes - defined by the glucose tolerance test - was present in significantly fewer of the men who had received testosterone than those who had received placebo. The testosterone group also showed a greater average improvement in their two-hour glucose reading. Put simply, adding testosterone to a lifestyle programme reduced the likelihood of having type 2 diabetes at two years compared with the lifestyle programme alone. That is a legitimate and important finding - but how it should be interpreted is where care is needed.

Why this is not a green light for testosterone as a diabetes treatment

Read in full, the T4DM researchers were careful not to present testosterone as a new diabetes therapy, and there are several reasons why.

• Lifestyle was the foundation, not testosterone. Every man in the trial was already in a weight-loss and exercise programme. Testosterone was tested as an add-on to lifestyle change, never as a replacement for it, and the trial offers no support for the idea that an injection can substitute for losing weight and moving more.
• The benefit likely works largely through body composition. Testosterone reduces fat mass and builds muscle, and much of its effect on glucose is thought to flow from those changes - the same changes that weight loss, resistance training and better sleep also produce, without medication.
• There are real safety trade-offs. Testosterone thickens the blood, and in T4DM a greater proportion of men taking it developed a rise in haematocrit (the concentration of red blood cells), which can require monitoring, a dose change or stopping treatment. The trial was not designed or large enough to settle longer-term questions about cardiovascular or prostate safety.
• It is not an approved diabetes medicine. Testosterone is not registered or recommended as a treatment for type 2 diabetes or for improving blood-sugar control. Clinical guidelines recommend testosterone only for men with genuine, symptomatic hypogonadism confirmed by repeat morning blood tests - not as a metabolic drug for men whose central problem is excess weight.

What this means for Australian men

If you have type 2 diabetes or are at risk of it, the proven foundations are unchanged: gradual weight loss, regular resistance and aerobic exercise, better sleep, and - where needed - diabetes medications prescribed by your GP. These are the levers with the strongest evidence, and they are also the same steps that can naturally lift testosterone in men whose levels have fallen alongside weight gain. Our guide on how to increase testosterone naturally covers them in detail, and testosterone, weight loss and body composition looks at how the two influence each other.

Testosterone therapy is a separate question. In Australia it is a Schedule 4 (prescription-only) medicine, prescribed only where it is clinically appropriate and supported by pathology evidence of deficiency. If you have symptoms that could point to low testosterone - persistent fatigue, low mood, reduced libido or loss of muscle - the sensible first step is a proper assessment with an AHPRA-registered GP, including morning blood tests, rather than seeking testosterone to manage your blood sugar. For men with confirmed deficiency, treating that deficiency can be worthwhile in its own right; for many others, the metabolic improvements they are hoping for come from the lifestyle changes underneath.

The bottom line

The Australian T4DM trial showed that, in overweight men with lower testosterone and impaired glucose metabolism, adding two years of testosterone to a lifestyle programme reduced the proportion who had type 2 diabetes compared with the programme alone. That is a real, carefully demonstrated effect - but testosterone is not a diabetes drug. It carries trade-offs such as a rise in haematocrit, it was tested on top of weight loss and exercise rather than instead of them, and it is not approved for controlling blood sugar. For most men, the foundations of managing glucose and supporting healthy testosterone are one and the same. Testosterone therapy has a genuine role for men with properly diagnosed deficiency, and that is a decision to make with a GP based on your own results.

References

1. Wu FCW, et al. Identification of Late-Onset Hypogonadism in Middle-Aged and Elderly Men. N Engl J Med. 2010;363(2):123-135. https://pubmed.ncbi.nlm.nih.gov/20554979/
2. 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. https://pubmed.ncbi.nlm.nih.gov/33338415/
3. Bhasin S, et al. Testosterone Therapy in Men With Hypogonadism (Endocrine Society clinical practice guideline). J Clin Endocrinol Metab. 2018;103(5):1715-1744. https://pubmed.ncbi.nlm.nih.gov/29562364/

This article is general information only and is not a substitute for personalised medical advice. Testosterone is a prescription-only (S4) medicine in Australia and is prescribed only where clinically appropriate. Speak with an AHPRA-registered GP about your individual situation.