The European Male Ageing Study surveyed 3,369 men aged 40 to 79 across eight European centres, measured morning testosterone by mass spectrometry, and asked which symptoms moved with the number.¹

Three held: poor morning erection, low sexual desire, and erectile dysfunction. The more of them a man reported, the lower his testosterone tended to be. Fatigue, depression and being unable to manage vigorous activity were also related to the level, but they did not form a syndrome with it. They are common in men whose testosterone is entirely normal.

That distinction carries most of the weight in this article. Low drive, flat mood and a gym session that costs more than it used to are the symptoms men actually present with, and they are the symptoms least able to tell you whether testosterone is the problem. A man who has those three and none of the sexual symptoms is the man whose testosterone is most likely to be blamed for something else. It is the same trap as why your bloods are normal but you feel terrible, arriving from the other direction: here the number is abnormal and the attribution is still wrong.

Testosterone runs on a daily rhythm and responds to what happened the night before, so when the blood is drawn changes the answer. The Endocrine Society guideline asks for a fasting morning total testosterone as the initial test, then a second morning sample to confirm it before anything is called a deficiency.² It also asks that the diagnosis be made only in men who have both the symptoms and the consistently low readings, not one or the other. Where a result sits inside the range but low, the question shifts to optimal versus reference range.

Total testosterone is not the whole picture either. Most of it travels bound to sex hormone binding globulin and is unavailable to tissue. Where the total sits near the bottom of the range, or where something is known to move SHBG, the guideline asks for a free testosterone by equilibrium dialysis or a validated calculation.² Thyroid status moves SHBG directly, which is one of the routes by which a thyroid problem arrives at the clinic looking like a testosterone problem.³

Thyroid. Primary hypothyroidism is associated with hypogonadotropic hypogonadism, and it is reversible. Free testosterone is reduced in men with primary hypothyroidism, and thyroid hormone replacement normalises it.³ Thyroid hormone also sets SHBG, so a thyroid problem can move both the free and the total in the same man for two different reasons. The presentation overlaps closely with being tired and cold with a normal thyroid result.

Sleep. One week of restricted sleep lowers daytime testosterone in healthy young men.⁴ The timing matters as much as the total: sleep restriction appears to lower testosterone when it falls in the first half of the night.⁵ This is the driver most often missed in men who are performing well, because short sleep is the thing they have chosen and stopped questioning. The wider case for protecting it sits in deep sleep and recovery.

Sleep apnoea, with a caveat worth knowing. Low testosterone in men with obstructive sleep apnoea relates to the obesity that accompanies it rather than to the apnoea, and it improves with weight loss.⁵ A meta-analysis of seven studies found CPAP made no significant difference to total testosterone, free testosterone or SHBG.⁶ Treating the breathing is worth doing for its own reasons. It is not a testosterone strategy.

Insulin and visceral fat. Plasma testosterone correlates directly with insulin sensitivity, and lower total testosterone together with lower SHBG predicts a higher incidence of metabolic syndrome.⁷ In a population sample of older men, total testosterone and SHBG were both inversely associated with metabolic syndrome after adjustment.⁸ Fasting insulin usually moves years before glucose does, which makes it the more useful early marker. The mechanism is the one set out in insulin resistance in women over 40, and it is not sex-specific.

Training that outruns intake. In endurance-trained men, low energy availability correlates with reduced testosterone alongside lower bone density and a lower resting metabolic rate.⁹ The man training hard on an unchanged or falling intake can present with exactly the picture he came in describing, and more training makes it worse. This sits alongside the ordinary reasons recovery takes longer than it used to.

An endocrinologist writing on sleep and testosterone put the clinical conclusion plainly: testosterone treatment is unlikely to benefit men whose low reading is secondary to obesity or depression, as against managing the underlying abnormality.⁵ The number is real. It is also downstream. Finding what sits above it is the job of a comprehensive executive health check.