Free T3 is a blood test that measures unbound triiodothyronine, the active thyroid hormone, most of which is made from T4 in the tissues. Practitioners use it when TSH is low, when a client is restricting energy or training hard, or when symptoms persist with normal TSH and free T4. Calorie restriction, illness and biotin can all shift the result.
| Test type | Measured in a blood sample |
|---|---|
| Measurements required | Free T3 only |
| Sample and collection | Blood draw at a collection centre |
| Preparation | Free T3 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 T3 reference and optimal ranges?
| Typical lab reference | Vively optimal | |
|---|---|---|
| Adults | 3.1 to 6.8 pmol/L | 4.0 to 5.5 pmol/L |
Free T3 ranges vary by method and some Australian labs cap the range near 6.0 pmol/L, so read results against the range on the client's report (RCPA guidance). The optimal band is a follow-up target, not a diagnostic cut-off.
Check a client's Free T3 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 T3 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 T3, 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 T3?
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 4.0 to 5.5 pmol/L band sits inside the 3.1 to 6.8 pmol/L lab range and works best as a flag for conversion and energy status.
Long-term calorie restriction lowers T3 while TSH and free T4 stay normal (Fontana et al., 2006). The IOC consensus on relative energy deficiency in sport lists low T3 as a marker of low energy availability.
Acute illness also lowers T3. A result under 4.0 in a client who is eating little or training heavily points to fuelling and recovery before anything thyroidal.
Above 5.5 with a low TSH warrants a closer look for T3-predominant hyperthyroidism. The band is a Vively clinical convention with no published threshold behind it.
What should Free T3 be read with?
- LowLow free T3, normal TSH and free T4, low intake or heavy training
Pattern of low energy availability or recent illness; address fuelling and retest.
- HighHigh free T3, suppressed TSH, normal free T4
Possible T3 toxicosis, an early hyperthyroid pattern; refer to the GP.
- LowLow free T3 with low ferritin or selenium
Iron and selenium both support T4 to T3 conversion; correct deficiency with the GP and retest.
- ContextNormal free T3, high TSH, low free T4
Hypothyroid pattern; free T3 is often the last to fall, so a normal value does not exclude it.
- HighHigh free T3 on liothyronine or desiccated thyroid
Usually reflects dose timing; collect before the dose and discuss with the prescriber.
What can affect a Free T3 result?
- Calorie restriction, long fasts or very low carbohydrate eating in the days before
- Acute or recent illness, surgery or major stress
- Biotin, which can falsely raise free T3 on some immunoassays
- Thyroid medication within hours of the draw, especially liothyronine
- Glucocorticoids, high-dose propranolol and amiodarone, which reduce T4 to T3 conversion
When should you refer the client to their GP?
- Free T3 above range with a suppressed TSH
- Palpitations, tremor, weight loss, heat intolerance or anxiety with any high free T3
- Low free T3 with missed periods, rapid weight loss or signs of restrictive eating
- Low free T3 with an abnormal TSH
How should a client prepare for a Free T3 test?
Copy or print this to send to your client.
Preparing for your Free T3 test
- Free T3 doesn't need fasting. If your request also includes glucose or insulin, fast for 8 to 12 hours before a morning collection.
- Avoid crash dieting before your test, as a sharp drop in food intake can lower T3.
- Pause any biotin supplement (often found in hair, skin and nail formulas) for at least 2 days before your test.
- 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.
- 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 T3 with clients?
Before reading a low free T3 as a thyroid problem, check energy and carbohydrate intake, training load and recent illness, and retest 4 to 6 weeks after fuelling is restored. Iron and selenium status are worth checking alongside.
Free T3 FAQs
Can a naturopath, nutritionist or dietitian order free T3 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.
Is free T3 worth ordering for hypothyroid screening?
Rarely on its own. In primary hypothyroidism, TSH rises and free T4 falls well before free T3 drops, so free T3 adds little at screening. It earns its place when TSH is low, when a client takes liothyronine, or when energy deficit or illness may be slowing conversion.
How fast does free T3 recover once a client eats enough?
Usually within weeks. Free T3 has a half-life of about a day and responds to energy intake within days, so a retest 4 to 6 weeks after intake and training have settled is a fair window. A free T3 that stays low after that, or a moving TSH or free T4, needs GP review.
Why is a client's free T3 low when TSH and free T4 are normal?
A low free T3 with normal TSH and free T4 most often reflects low energy availability, recent illness or reduced T4 to T3 conversion rather than thyroid disease. Check intake, carbohydrate, training load and iron and selenium status, then retest 4 to 6 weeks after fuelling is restored. A low free T3 with an abnormal TSH needs GP review.
Can a normal free T3 rule out hypothyroidism?
No. Free T3 is often the last of the three to fall, so a normal value does not exclude hypothyroidism. TSH and free T4 are the better screen when the pattern fits, a high TSH with a low free T4 needs GP review whatever the free T3 shows.
References
- Royal College of Pathologists of Australasia. RCPA Manual: Free triiodothyronine. Source
- Pathology Tests Explained. Thyroid function tests.
- Fontana L, et al. Effect of long-term calorie restriction with adequate protein and micronutrients on thyroid hormones. Journal of Clinical Endocrinology and Metabolism. 2006.
- Mountjoy M, et al. IOC consensus statement on relative energy deficiency in sport (RED-S): 2018 update. British Journal of Sports Medicine. 2018.