Free T4 is a blood test that measures unbound thyroxine, the main hormone the thyroid releases. Practitioners use it when TSH looks off, when symptoms persist despite a normal TSH, or to separate a thyroid problem from a pituitary one. It is method-dependent, and biotin, recent levothyroxine or heparin can falsely raise the result on some assays. Practitioners can order it through the Vively practitioner platform.
| Test type | Measured in a blood sample |
|---|---|
| Measurements required | Free T4 only |
| Sample and collection | Blood draw at a collection centre |
| Preparation | Free T4 doesn't need fasting. If your request also includes glucose or insulin, fast for 8 to 12 hours before a morning collection. |
| Turnaround | About 3 to 5 business days |
| Available through | Thyroid Function Tests. Available in: Full Body Health Check, Weight Insight Panel, Weight Management Test, Female Hormone Health Test, Perimenopause Test, Menopause Test. |
| Pricing | Price on sign-up |
| Results | Returned in the Vively practitioner platform, beside your client's other markers |
What are the Free T4 reference and optimal ranges?
| Typical lab reference | Vively optimal | |
|---|---|---|
| Adults | 10 to 25 pmol/L | 12 to 22 pmol/L |
Free T4 is method-dependent and several Australian labs report upper limits nearer 19 to 20 pmol/L, so read the result against the range printed on the client's report (RCPA guidance). The optimal band guides follow-up and is not a diagnostic cut-off.
Check a client's Free T4 result
Enter the value from your client's lab report to see where it sits against the typical lab range and the Vively optimal range.
A guide only, not a diagnosis. One result is a starting point: read it with symptoms, history and a repeat test. Labs differ, so check the result against the range on your client's report. Nothing you enter is stored or sent.
How do you arrange Free T4 testing through Vively?
- Get verified. Practitioners with a recognised qualification or membership (ATMS, NHAA, ANTA, Dietitians Australia, NSA, CMA or Ahpra registration) can request free practitioner access.
- Choose a panel. Log in to the practitioner platform and pick a panel that includes Free T4, or build your own. Add-on pricing shows when you sign up.
- Client collects. Your client collects at a partner collection centre, with their request on their phone or printed.
- Review results. Results come back to you in the practitioner platform, beside your client's other markers, ready for the follow-up consult.
Which Vively panels include Free T4?
Log in to the practitioner platform to view a panel in full, or build and save your own from 100+ tests and panels.
- Log in to view this panelWeight Insight Panel8 markers · Heart & metabolicPrice on sign-up
- Female Hormone Health Test17 markers · HormonesPrice on sign-up
Two blood tests timed to the cycle, to show how hormones shift across it.
Log in to view this panel - Perimenopause Test17 markers · HormonesPrice on sign-up
Two blood tests timed to the cycle: Day 2 to 5 (ideally Day 3), then around Day 21 or 7 days before the next period.
Log in to view this panel
Why does the optimal range sit there?
Our 12 to 22 pmol/L band trims both tails of the 10 to 25 pmol/L lab range, where results most often travel with an abnormal TSH.
A free T4 of 10 to 12 pmol/L with TSH drifting above 2.5 mIU/L is an early pattern worth repeating, especially with positive thyroid antibodies.
The Rotterdam Study (Heeringa et al., 2008) linked high-normal free T4 with higher atrial fibrillation risk in older adults, even with a normal TSH.
No published threshold sits behind the band, and it has to sit inside the lab's method: if a lab caps the range at 20 pmol/L, a result of 21 is out of range.
What should Free T4 be read with?
- LowLow free T4, high TSH
Primary hypothyroid pattern that needs GP review.
- HighHigh free T4, suppressed TSH
Hyperthyroid pattern; refer promptly.
- HighHigh free T4 with normal or high TSH
Most often assay interference (biotin, heterophile antibodies) or levothyroxine taken just before the draw; rarely a pituitary cause, so repeat and involve the GP.
- LowLow free T4 with low or normal TSH
Possibly a pituitary cause or severe non-thyroidal illness; needs GP review.
- LowLow-normal free T4 with low ferritin
Iron deficiency is often seen with lower thyroid hormone output; correct it with the GP and retest.
What can affect a Free T4 result?
- Biotin, which can falsely raise free T4 on some immunoassays
- Levothyroxine taken within a few hours before collection
- Heparin, including recent injections, which can falsely raise free T4 in the sample
- Pregnancy, which needs trimester-specific ranges
- Acute illness, amiodarone, and anticonvulsants such as phenytoin and carbamazepine
- High-dose iodine or kelp supplements
When should you refer the client to their GP?
- Free T4 below the lab range with a high TSH
- Free T4 above range with a suppressed TSH, or with palpitations, tremor, weight loss or eye changes
- Low free T4 without a raised TSH (possible pituitary cause)
- Any abnormal free T4 in pregnancy or when planning pregnancy
How should a client prepare for a Free T4 test?
Copy or print this to send to your client.
Preparing for your Free T4 test
- Free T4 doesn't need fasting. If your request also includes glucose or insulin, fast for 8 to 12 hours before a morning collection.
- If you take thyroid medication, have your blood taken before your morning dose and take it straight afterwards, unless your doctor has told you otherwise.
- Pause any biotin supplement (often found in hair, skin and nail formulas) for at least 2 days before your test.
- Take your pathology request to any of 4,000+ collection centres. Bring it printed or on your phone.
- Keep taking prescribed medicines as usual unless your doctor tells you otherwise.
- Results usually take about 3 to 5 business days and come to your practitioner. Book your results appointment for after that.
How do practitioners use Free T4 with clients?
Iodine is needed to make thyroxine, and selenium and iron support the enzymes that make and convert it. Check intake (dairy, seafood, iodised salt in bread) and iron status before reading a low-normal free T4 as anything more complex, then retest with TSH after 3 months of dietary change.
Free T4 FAQs
Can a naturopath, nutritionist or dietitian order free T4 through Vively?
Yes. Add it to any client panel on the Vively practitioner platform. It sits in Thyroid Function Tests and is also in Full Body Health Check, Weight Insight Panel and Weight Management Test. Add-on pricing shows when you sign up. No account yet? Request free access.
Should a client take levothyroxine before a free T4 test?
No. Ask them to collect before the morning dose and take it straight after. Levothyroxine raises free T4 for several hours after ingestion, which can make a well-dosed client look over-replaced. TSH is less affected by timing, and any dose change stays with the prescribing doctor, who reads TSH first.
Is free T4 useful when TSH is normal?
Sometimes. In a healthy client a normal TSH usually means a normal free T4, so routine pairing adds little. Add free T4 when TSH is near either end of its range, when symptoms are strong, when a pituitary problem is possible, or when the client takes thyroid medication.
What does a low-normal free T4 with a TSH around 3 mIU/L tell me?
It can be an early pattern worth watching. A free T4 of 10 to 12 pmol/L with TSH drifting above 2.5 mIU/L is lower than the Vively optimal range of 12 to 22 pmol/L, though both may still sit inside the lab range. Check iron and iodine intake, retest in about 3 months, and ask the GP about antibodies if it persists.
Why might free T4 be high when TSH is normal?
Most often it is assay interference or timing rather than disease. Biotin, heterophile antibodies, recent heparin or levothyroxine taken just before the draw can all raise free T4. Repeat the test with these removed. A persistently high free T4 with a normal or high TSH needs GP review, as a pituitary cause is rare but possible.
References
- Royal College of Pathologists of Australasia. RCPA Manual: Free thyroxine. Source
- Pathology Tests Explained. Thyroid function tests.
- Heeringa J, et al. High-normal thyroid function and risk of atrial fibrillation: the Rotterdam Study. Archives of Internal Medicine. 2008. Source
- US Food and Drug Administration. The FDA warns that biotin may interfere with lab tests. 2017.