A low testosterone result: what to examine first
Two 46-year-olds get the same number back, below the reference range. Almost nothing else about it is the same. A low value is a finding, not a diagnosis.
Two 46-year-olds get the same result back: total testosterone under the reference range. One has slept six broken hours a night for two years and gained twelve kilos. The other sleeps well and trains sensibly. The number is identical. Almost nothing else about it is.
A low value is a finding, not a diagnosis. By the end of this article you will know the order of questions that turns one into the other.
Why is one low reading not enough?
Testosterone is highest in the morning and drifts down all day. An afternoon blood draw can put a perfectly ordinary man below the range. Add the noise of biology: a bad night, a heavy week of training, a virus coming on. A meaningful share of men with one low result are back in range on a repeat test.
A low testosterone should be confirmed on a second morning sample before it is allowed to mean anything. That habit removes more false conclusions than any other step here.
Then there is what the number actually measures. Most circulating testosterone is bound to carrier proteins and unavailable. The main carrier is sex hormone binding globulin, or SHBG. Only a small free fraction does the work. SHBG is not a constant. It falls with obesity and insulin resistance, and rises with age, thyroid excess and liver disease.
When SHBG shifts, the total moves with it while the free fraction may not. That is why the total misleads most often in exactly the people most likely to be tested. Free testosterone calculated from total, SHBG and albumin is the better guide. Reference ranges are not universal either: the same blood on two laboratory methods can straddle a cut-off.
Suppose a value survives all that. The next question is not what to do about it. It is where in the chain it comes from.
Which measurement splits the causes in two?
Testosterone sits at the end of a three-part chain. The hypothalamus, a control centre in the brain, releases a pulse. The pituitary answers with luteinising hormone, or LH. LH instructs the testis to produce. Testosterone then feeds back upstream and quietens the pulse. It is a thermostat with the boiler at the bottom.
So when testosterone is low, one measurement says which part failed. A raised LH means the instruction is loud and the testis is not answering. A low or unremarkable LH beside a low testosterone means the instruction never came. Doctors call the first primary and the second secondary. The second is misread constantly, because a mid-range LH looks normal until you notice it should have risen.
Return to the man who sleeps badly and has gained twelve kilos. His LH will usually be low, or unremarkably mid-range. His problem is upstream, and upstream is where most of the interesting causes live.
What quietly turns the signal down?
Sleep first. Much of testosterone release is tied to the night’s deeper stages. Restricting healthy young men to about five hours a night for one week measurably lowers their daytime levels. Untreated obstructive sleep apnoea does the same thing nightly, for years, and is missed constantly.
Body fat is the second driver, and it works in two directions at once. Fat tissue contains aromatase, an enzyme that converts testosterone into oestradiol, a form of oestrogen. Oestradiol brakes the pulse upstream. Weight gain lowers testosterone and strengthens the very signal that suppresses its production. The fall in SHBG alongside insulin resistance makes the total look worse still.
The rest of the list is worth knowing. Sustained energy shortage, from restricted eating or a training load the body is not fed for, switches reproductive output down. Reproduction is expensive, and the body treats it as optional. Opioid painkillers suppress the pulse strongly and are the most under-recognised cause. Alcohol and prolonged stress do quieter versions of the same.
All of these can move. What makes the sequence urgent is that low testosterone does not sit still while they are examined.
Why does the order of investigation matter?
Testosterone helps hold muscle. Less of it means less lean mass, and lean mass is where most glucose is disposed of. Losing muscle therefore worsens insulin resistance. Worse insulin resistance travels with more fat, more aromatase and more oestradiol, which drops the signal from above further still.
Low testosterone, muscle loss, insulin resistance and further suppression form a loop that tightens on itself. That is why the order in which things are examined changes the outcome. Sleep, apnoea, energy intake and training load loosen the loop from several points at once. Address them early and whatever comes next works on cleaner ground.
Which brings the question most people arrive with.
What does replacement do, and what does it not?
Start with the misconception worth flipping. A decline of roughly one per cent a year from the thirties onward is a normal feature of ageing, not a disease. An age-appropriate drift, with no symptoms and no confirmed deficiency, is not a finding in need of correction. Genuine deficiency is another matter: repeated low morning values, consistent symptoms, and a cause looked for.
Proven: in confirmed deficiency, randomised trials show symptomatic benefit, most consistently for sexual function. The large randomised safety trial reported in 2023 found no excess of major cardiovascular events, settling a decade-long argument. Promising: effects on mood, energy, lean mass and bone density lean positive in deficient men, and researchers are refining who benefits most. Practical guidance: replacement turns the body’s own production and sperm output down while it runs, so fertility plans belong in the conversation before starting, and recovery after stopping can take months. Blood count is monitored routinely, because a rise in red cells is a recognised effect.
Replacement relieves the symptoms of true deficiency, and choosing the right person for it is most of the skill. Whether it is right for any one man is a decision for him and his own doctor, made with the whole picture in view.
Three answers are worth having before a low testosterone is interpreted. What time was the sample drawn? What did the last three months of sleep look like? And what was LH doing in the same tube?
- One low reading is a finding, not a diagnosis. Confirm it on a second morning sample.
- SHBG changes make the total misleading in obesity, insulin resistance and ageing. Calculated free testosterone guides better.
- LH splits the causes. Raised points to the testis, low or unraised points to the signal above.
- Most secondary cases sit on something that can move: sleep, apnoea, weight, energy, opioids, alcohol.
- A normal age-related drift is not deficiency. Replacement pauses natural production, so raise fertility plans first.
Reading a hormone as the end of a chain rather than a verdict is the habit this field is built on. The Hormones knowledge check will show you how firmly it has settled.
