Total testosterone is usually measured in the morning, between 07:00 and 10:00, and preferably fasting. If it is low, it is normally repeated on another day before treatment is considered. One result does not diagnose hypogonadism: it must be reproducible, fit the symptoms and be investigated for a cause.
How should testosterone be tested?
Testosterone changes during the day and is generally higher in the morning. It can also fall temporarily during illness, after poor sleep or with major calorie restriction. The EAU therefore recommends morning testing, generally between 07:00 and 10:00, fasting where possible.
If the result is low, it should normally be repeated before TRT is considered. If you are acutely unwell, recovering from surgery or going through a major health change, waiting until you are stable is often more informative.
What does 12 nmol/L mean?
The EAU uses about 12 nmol/L as a practical threshold in late-onset hypogonadism. This is approximately 346 ng/dL.
It is not a magic boundary. The result must be repeated and interpreted alongside symptoms, age, SHBG, other illnesses and the laboratory method. A man at 11.8 nmol/L is not automatically diagnosed, and a man at 12.2 nmol/L is not automatically excluded if the wider picture warrants further assessment.
What tests may be added if the result is low?
LH and FSH
Pituitary hormones that help distinguish a mainly testicular problem from insufficient signalling from the brain.
SHBG and free testosterone
Useful when SHBG may be abnormal and total testosterone does not reflect available testosterone well.
Prolactin
Can be useful when a pituitary cause is suspected, libido is markedly reduced or the hormonal pattern suggests it.
Full blood count / haematocrit
Important before TRT because testosterone can increase the proportion of red blood cells.
Thyroid tests, iron or ferritin, liver tests, glucose, lipids and other investigations may be added according to the history.
What is the difference between total testosterone, free testosterone and SHBG?
Most testosterone circulates bound to proteins, mainly SHBG and albumin. Only a small fraction is free. If SHBG is unusually high or low, two men with the same total testosterone can have different available testosterone.
Free testosterone can be measured or, more commonly, calculated using total testosterone, SHBG and albumin. Interpretation depends on the method, so one universal cut-off should not be applied to every laboratory.
What can lower testosterone without permanent damage?
- Obesity and metabolic syndrome.
- Acute or systemic illness.
- Insufficient sleep and some sleep disorders.
- Opioids, glucocorticoids and other medicines.
- Current or previous anabolic-steroid use.
- Severe energy restriction, rapid weight loss or extreme exercise.
In some of these situations the fall is functional and may partly reverse. That is why finding the cause matters as much as measuring the number.
If I already use testosterone, when should the blood test be taken?
During TRT, a result only makes sense if you know when the sample was taken relative to the last dose. Different formulations have different profiles.
- Daily gel: timing depends on the product. Some product information recommends measuring before the dose and some a few hours after application. Follow the instructions for the specific gel and your clinician.
- Long-acting testosterone undecanoate injection: levels are measured just before the next injection.
Do not change the dose yourself based on an isolated result.
Source: AEMPS CIMA product information.
A blood test is not the diagnosis
The useful question is not only "what is my level?" but "is this result reproducible, does it explain my symptoms and do we know why it is low?"