The age-related decline in testosterone is an established biological phenomenon, but that does not mean every man will benefit from therapy. TRT has the strongest evidence base for sexual function in men with established hypogonadism, and there are initial indications of a reduced risk of diabetes. For cognition and physical performance, however, randomised trials show no clinically relevant benefit. Consult a doctor for a thorough diagnosis before proceeding to treatment.
Testosterone does indeed decline in men with age, and this is firmly supported by large-scale longitudinal research. The decline begins on average around the age of 35 to 40 and amounts to roughly 1 to 3 percent per year. Above the age of 60, approximately one in five men has levels that are formally considered too low; above the age of 80, that proportion rises to half. The decline occurs independently of chronic illness or medication use. Notably, in addition to the ordinary age-related decline, a separate, as yet unexplained population-wide decrease has been found in American men that is greater than can be explained by aging alone. Possible causes such as obesity or smoking do not fully account for this.
Lower testosterone is associated in observational studies with greater muscle loss, increased fat accumulation, a worse cardiovascular profile, a higher risk of metabolic syndrome, and possibly an increased chance of dementia. Nevertheless, the causal direction is not always clear: it may be that poorer overall health suppresses testosterone, rather than the other way around. For many of these outcomes, lower testosterone should for now be regarded as a signal, not as a proven cause.
Testosterone replacement therapy (TRT) has the strongest evidence for improving sexual function and quality of life in men with established hypogonadism. In a randomised trial involving 788 men aged 65 and older, sexual function improved significantly after one year of therapy. Another randomised trial in more than a thousand men aged 50 to 74 also suggested that TRT may reduce the risk of type 2 diabetes in men with an increased waist circumference and low testosterone. The latter is a promising but as yet isolated finding that warrants confirmation.
For cognition and physical performance, the picture is different. Despite the observational associations between low testosterone and cognitive decline, randomised trials show no improvement in cognitive function following TRT. For mobility and muscle function, the effects are limited or absent, and the clinical relevance is unclear. The idea that TRT halts or reverses muscle loss or memory decline in later life is therefore not clinically proven.
Anyone considering doing something about their testosterone levels should first know that a low result does not automatically mean 'normal aging'. Low testosterone can also point to a condition of the pituitary gland or hypothalamus, and a medical work-up is necessary before therapy is started. Strength training produces an acute, temporary rise in testosterone, but whether that yields structural health benefits via testosterone has not been demonstrated on the basis of the available research.
Based on multiple large-scale longitudinal cohort studies and randomised trials, including the Testosterone Trials (n=788, 65+) and an Australian RCT (n=1007, 50-74 years). The observational associations are consistent, but causality for many outcomes has not been proven.