Low testosterone: what to test before you conclude anything
A single total testosterone number is the most over-interpreted result in men's health. It moves with the time of day, the season, a poor night's sleep and an acute illness — and on its own it cannot tell you why it is low.
What people notice
- Flat mood and motivation rather than sadness
- Reduced morning erections and libido
- Training as hard as ever with less to show for it
- Persistent fatigue that sleep does not fix
- Gradual gain around the waist despite unchanged habits
Symptoms overlap between causes. This list is a prompt to measure, not a diagnosis.
Markers worth measuring
- Total testosterone
- The headline number — only meaningful on a 7am–10am sample, and worth repeating before anything is concluded from it.
- SHBG and albumin
- Free testosterone is calculated from these. High SHBG can leave total testosterone looking normal while the free fraction is genuinely low.
- LH and FSH
- Separates a testicular cause (high LH) from a pituitary or hypothalamic one (low or normal LH with low testosterone).
- Sensitive estradiol
- Testosterone aromatises to estradiol; the ratio matters. Standard assays are unreliable in men — the sensitive method is the one to use.
- Prolactin
- Elevated prolactin suppresses the axis and is a recognised, checkable cause of low testosterone.
- TSH, free T4, free T3
- Thyroid dysfunction produces almost the identical symptom set and frequently coexists.
- Ferritin and iron studies
- Iron deficiency causes the same fatigue and low drive and is common in endurance athletes and blood donors.
- Fasting insulin and HbA1c
- Insulin resistance and visceral fat suppress testosterone. This is the most common reversible driver in men under 50.
Why timing dominates. Testosterone follows a daily rhythm and peaks in the first hours after waking. The reference ranges laboratories use assume a morning sample, so an afternoon draw can read 20–30% lower for no biological reason at all. Any result that matters should be a fasting morning sample, and any low result should be confirmed on a second morning sample on a different day before it is treated as real.
Why the context markers matter more than the headline. Low testosterone is frequently downstream of something else: sleep apnoea, iron deficiency, thyroid dysfunction, insulin resistance, chronic inflammation, heavy alcohol use, or a sustained calorie deficit. A hormone-only screen finds the low number and stops. A broad panel finds the reason the number is low, and the reason is often something you can address.
The SHBG trap. Two men with an identical total testosterone of 14 nmol/L can have very different free fractions if their SHBG differs. High SHBG rises with age, thyroid excess, liver disease and low body fat; low SHBG tracks with insulin resistance and obesity. Without SHBG and albumin, free testosterone cannot be calculated, and the interpretation is guesswork.
What FORM does and does not do. We are a pathology testing service. We measure the panel, and our medical lead writes a plain-English report explaining each marker and what sits outside range. That report is health information, not a diagnosis, and it is not advice to start any treatment. Decisions about medicines belong to a doctor registered where you live, after their own assessment.
FAQs
- What time should a testosterone test be taken?
- Between 7am and 10am, fasted if the panel also includes glucose, insulin or lipids. This is what the reference ranges assume.
- Is one low result enough to act on?
- No. Guidelines everywhere recommend confirming a low testosterone on a second morning sample, because a single reading can be depressed by illness, poor sleep, a hard session or mistiming.
- Why measure estradiol in men?
- Testosterone converts to estradiol, and both very high and very low levels cause symptoms. Use a sensitive assay — the standard immunoassay is not accurate at male concentrations.
- Can low testosterone be caused by something fixable?
- Often, yes. Sleep apnoea, iron deficiency, thyroid dysfunction, insulin resistance and severe under-eating all suppress the axis. That is exactly why the panel is broad.
Measure it properly, in one morning draw.
Related testing topics
- Cortisol and chronic stress: what a blood test can and cannot show
- Thyroid testing for fatigue: why TSH alone is not enough
- Male fertility testing: what to measure and in what order
- Iron, B12 and vitamin D: the deficiencies behind most fatigue
- Metabolic health testing: HbA1c, fasting insulin and what they miss
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