How to prepare for a testosterone blood test — Australia
A practical patient guide. The result depends on when and how the blood is taken, not just what's in it. Information only — your GP gives the specific instructions for your test.
The single most important thing: testosterone is normally highest in the morning and falls during the day. An afternoon test can show a falsely low result. The Endocrine Society of Australia recommends testing between 8:00 am and 10:00 am.
The four things that matter
1Timing: 8:00 am to 10:00 am
Testosterone follows a daily rhythm — peaks in the early morning, drops through the day. An afternoon sample can be 20–25% lower than the same person's morning level in younger men (the difference is smaller, around 10%, in older men) — but timing still matters at any age. The Endocrine Society of Australia recommends collection between 8:00 am and 10:00 am.
- Book the earliest morning appointment your lab offers.
- For shift workers, "morning" means shortly after you wake — not 8 am clock time. Tell the GP and the lab if you work nights so the timing is set to your sleep cycle.
- If you can't make it before 10 am for any reason, tell your GP — the result needs to be interpreted in light of when it was taken.
Source: Endocrine Society of Australia (MJA 2016).
2Fasting: usually yes
Eating before the blood draw can lower the testosterone result. Most labs request an 8-hour overnight fast for testosterone testing. Water is fine — drink it.
- OK during the fast: water, prescribed medications (unless your GP has said otherwise).
- Avoid: coffee, tea, food, juice, lollies, gum, alcohol.
- If you're diabetic or on a medication that's hard to take fasted, ask your GP before the test for a plan.
- The easiest way is to book early morning and have the test before breakfast — eat straight afterwards.
Source: Endocrine Society Clinical Practice Guideline; Endocrine Society of Australia (MJA 2016).
3You'll usually need two morning tests, not one
This is the part patients are most often surprised by. About 30% of men with an initial low testosterone result have a normal result on a repeat test. International and Australian guidelines therefore require two separate morning fasting samples, both confirming low levels, before low testosterone is diagnosed.
- The two samples are usually taken on separate mornings — your GP or lab will let you know when to come back for the second one.
- Same conditions both times: morning, fasting, well-rested.
- This is why your GP may book you in for a "repeat" even if the first result was clearly low.
Source: Endocrine Society of Australia (MJA 2016); Endocrine Society Guideline.
4What else gets tested with testosterone
For an initial low-testosterone workup, the GP usually adds a few related tests to the same blood draw — they all need the same morning, fasted conditions:
- LH and FSH — hormones from the pituitary gland that tell the testes to make testosterone. These help work out where any problem is.
- SHBG (sex hormone–binding globulin) — needed to calculate "free testosterone" if the total result is borderline.
- Prolactin — added if your LH/FSH results suggest the problem may be in the pituitary gland.
- Other tests (PSA, full blood count, lipids, HbA1c) may be added depending on your symptoms — your GP decides what's relevant.
Source: Endocrine Society of Australia (MJA 2016).
Things that can affect the result — postpone if any apply
Are you acutely unwell?
Acute illness (flu, infection, recent hospital admission, recent surgery) temporarily lowers testosterone — sometimes dramatically. The result won't reflect your real baseline. Postpone the test until you've recovered — ask your GP how long to wait, as this varies by illness and severity.
Source: Endocrine Society of Australia (MJA 2016).
Sleep, stress, exercise the day before
- Poor or short sleep in the lead-up can lower testosterone — studies show a measurable drop after about a week of short sleep, so try to get decent rest in the days before, not just the one night.
- Heavy or unaccustomed exercise in the 24 hours before the test can lower testosterone temporarily — skip the hard session the day before.
- Major stress events can affect the result. Pick a relatively normal week if possible.
- If you're repeating the test, try to keep these conditions similar both times.
Source (sleep): Leproult & Van Cauter, JAMA 2011.
Medications and supplements
Several medications and supplements can affect testosterone results:
- Testosterone replacement, anabolic steroids, opioid pain medication, oral corticosteroids — all change testosterone levels significantly.
- Finasteride, dutasteride (used for hair loss or prostate) — affect related hormones.
- High-dose biotin supplements — can interfere with the laboratory assay itself (different from affecting actual testosterone). Most labs ask you to stop biotin 2–3 days before the test.
- Herbal supplements marketed for "men's health" or "testosterone boosting" — let your GP know.
Tell your GP everything you take — prescription, over-the-counter, supplements, recreational. None of it changes whether you get the test; it changes how the result is interpreted.
Source: Endocrine Society of Australia (MJA 2016); Endocrine Society Guideline.
After the test
Results usually take 1–3 working days. They go to the doctor who ordered the test. Book a follow-up appointment to discuss the result — testosterone numbers depend on the lab's reference range, your age, your symptoms, and what else was tested. The clinical interpretation is for your GP, not for the result printout alone.
If you've had a low result, your GP will arrange a second morning fasting sample before any treatment decisions are made.
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