Free T3 is triiodothyronine, the active form of thyroid hormone that sets the pace for your cells. Most of it is made in your tissues from T4, so free T3 shows thyroid hormone at work, and it can dip with heavy dieting, hard training, illness or ongoing stress even when TSH looks fine. The lab range is 3.1 to 6.8 pmol/L, and we aim for 4.0 to 5.5 pmol/L. You'll find free T3 in the Weight Management Test ($349) and the Perimenopause and Menopause Hormone Tests ($299 each), or in the Full Body Health Check, $499 including your Baseline. It has no Medicare rebate.
Test free T3 with TSH and free T4 in Vively's Full Body Health Check, or in our Weight Management, Female Hormone, Perimenopause and Menopause Hormone Tests. You start with your Baseline Health Check, which screens TSH.
Every marker in range? Get your $99 back. T&Cs apply.Want to save 25%? Try our 4-Month Reset insteadFree T3 (free triiodothyronine) is the unbound share of T3, the active form of thyroid hormone. It enters cells and attaches to thyroid hormone receptors to set the pace of your metabolism. Only about a fifth of your T3 comes straight from the thyroid. The rest is made in your tissues by converting T4.
Your thyroid mainly releases T4 (measured as free T4), a fairly inactive hormone. Enzymes called deiodinases in your liver, kidneys, muscles and brain strip one iodine atom from T4 to make T3. These enzymes need selenium, and they speed up or slow down with energy intake, illness and stress.
T3 lasts about a day in the blood, compared with about a week for T4, so free T3 responds faster to change.
Free T3 shows how much active thyroid hormone is available to your cells.
Your Baseline screens with TSH; free T4 is usually next. Free T3 comes later in the queue because, in an underactive thyroid, your body converts more T4 to protect T3, so it stays normal until late.
It is most useful:
Normal does not always mean optimal, though free T3 needs care here. A low-normal free T3 in someone eating very little or training hard is a sign to look at fuelling and recovery. It is not a reason for thyroid medication.
For adults, free T3 is usually 3.1 to 6.8 pmol/L.
| Range | |
|---|---|
| Typical lab reference range (adults) | 3.1 to 6.8 pmol/L |
| Vively optimal range | 4.0 to 5.5 pmol/L |
Free T3 ranges vary between lab methods, and some Australian labs use a lower range with an upper limit around 6.0 pmol/L, so check your report. Optimal ranges are what we aim for to support long-term health, not diagnostic cut-offs.
A high free T3 alongside a low TSH usually points to an overactive thyroid (hyperthyroidism), and your GP will look for the cause. If TSH is normal, a medication dose or lab interference is the likelier explanation.
Check these first:
Then thyroid conditions:
A low free T3 with a normal TSH and free T4 most often means your body is conserving energy during illness, dieting or heavy training. It usually corrects itself once the cause passes. Less often, it is part of an underactive thyroid (with high TSH and low free T4) or, rarely, a pituitary problem.
Common reasons:
Recent eating, illness, medication timing and biotin can all shift a free T3 result.
Do you need to fast? No. One overnight fast won't meaningfully change free T3, and you'll fast anyway with the Full Body Health Check because of glucose and insulin.
Free T3 makes sense only with TSH and free T4 beside it.
| Pattern | What it can mean |
|---|---|
| High TSH, low free T4, low or normal free T3 | An underactive thyroid. Free T3 is often the last to fall. Needs GP review. |
| Low TSH, high free T3, normal or high free T4 | An overactive thyroid, sometimes T3-predominant early Graves' or a nodule. Needs prompt GP review. |
| Normal TSH, free T4 and free T3, but ongoing symptoms | Thyroid hormone is unlikely to explain how you feel. Look at ferritin, vitamin D, glucose, sleep and stress. |
| Low free T3, normal TSH and free T4 | Usually illness, dieting or heavy training. Recheck when you're well and eating normally. |
| Low-normal free T3 with low ferritin or iron | Iron deficiency can affect thyroid hormone production and conversion. Correct the iron and recheck. |
Because so much T3 is made in your tissues, eating enough is the biggest everyday lever. Lifestyle won't fix thyroid disease, but it can stop you pushing free T3 down.
Book a GP visit if your free T3 sits outside your lab's range and your TSH is abnormal too, or if you have signs of an overactive thyroid such as a racing heart, weight loss, tremor, heat intolerance, anxiety or frequent bowel motions.
Seek prompt care for a racing or irregular heartbeat with chest pain or breathlessness, painful or bulging eyes, or a swollen neck. If you're pregnant, have any abnormal thyroid result reviewed quickly. A low free T3 with normal TSH and free T4 isn't urgent unless you've been unwell for a while or lost weight without trying.
Your Baseline Health Check screens TSH. Free T3 is added, together with free T4, in our Full Body Health Check, Weight Management Test, Female Hormone Health Test, and Perimenopause and Menopause Hormone Tests. The exact markers can depend on the referring doctor's review.
We arrange your referral through our GP network, and you give one blood sample at one of 3,700+ accredited partner collection centres. Your results land in the app, usually within a few days. Your practitioner reads free T3 against your TSH and free T4, and your eating and training, then builds a personalised plan. A retest shows whether a low reading recovers.
Check your free T3 with the Full Body Health Check.
No. An overnight fast doesn't change free T3 much, though you'll fast anyway if it's part of Vively's Full Body Health Check, which includes glucose and insulin. Avoid crash dieting in the days before, pause biotin if your doctor agrees, and ask your GP about thyroid medication timing.
TSH and free T4 answer most thyroid questions, so free T3 often isn't needed. In an underactive thyroid, free T3 usually stays normal until late, so it adds little. Doctors mainly add free T3 when TSH is low, to look for an overactive thyroid, or when monitoring certain treatments.
Free T4 is the main hormone your thyroid releases, and it is fairly inactive. Free T3 is the active hormone, mostly made from T4 in your tissues. Free T4 reflects thyroid output. Free T3 reflects active hormone available to cells and responds faster to illness, dieting and medication.
Slightly. It is under the 3.1 to 6.8 pmol/L range, but inside the range some labs use. If your TSH and free T4 are normal and you've recently been unwell, dieting or training hard, it most often reflects your body conserving energy. Retest once you're well and eating normally. If TSH is abnormal, see your GP.
Yes. Very low calorie diets, long fasts and very low carbohydrate eating can lower free T3 within days, because your body slows T4 to T3 conversion to save energy. It usually recovers when you eat enough again, which is one reason we favour steady, moderate weight loss.
Many blood tests can be Medicare-funded when a doctor orders them for a clinical reason. With Vively, we arrange the referral through our GP network, so you don't need a separate GP visit before your test. Your own GP can also order thyroid tests for a clinical reason, usually starting with TSH.
Among 1,636 Vively members tested in 2024 to 2026, women aged 40 to 49 had a median free T3 of 5.0 pmol/L (middle half 4.7 to 5.3). Men the same age: 5.5 pmol/L. The lab range is 3.1 to 6.8 pmol/L. A member median is not a target.
The line is the median member at each age. The shaded bands show where most members sit. The dashed red lines are the lab range.
| Group (pmol/L) | Members | Median | Middle 50% | 5th to 95th percentile | Lab range |
|---|---|---|---|---|---|
| Women 18 to 29 | 69 | 5.4 | 5.0 to 5.6 | 4.3 to 5.9 | 3.1 to 6.8 |
| Women 30 to 39 | 268 | 5.1 | 4.8 to 5.5 | 4.2 to 6.0 | 3.1 to 6.8 |
| Women 40 to 49 | 336 | 5.0 | 4.7 to 5.3 | 4.0 to 5.8 | 3.1 to 6.8 |
| Women 50 to 59 | 247 | 4.9 | 4.7 to 5.3 | 4.1 to 6.0 | 3.1 to 6.8 |
| Women 60 and over | 130 | 4.9 | 4.5 to 5.2 | 3.8 to 5.8 | 3.1 to 6.8 |
| Men 18 to 29 | 69 | 5.6 | 5.3 to 6.1 | 4.5 to 6.7 | 3.1 to 6.8 |
| Men 30 to 39 | 169 | 5.6 | 5.2 to 6.0 | 4.3 to 6.7 | 3.1 to 6.8 |
| Men 40 to 49 | 179 | 5.5 | 5.1 to 5.9 | 4.4 to 6.4 | 3.1 to 6.8 |
| Men 50 to 59 | 111 | 5.5 | 5.0 to 5.9 | 4.2 to 6.5 | 3.1 to 6.8 |
| Men 60 and over | 58 | 5.1 | 4.8 to 5.5 | 4.1 to 6.3 | 3.1 to 6.8 |
Pick your sex and age, then type your result.
What is a normal free T3 level for my age?
Among 1,636 Vively members (first blood test, January 2024 to October 2026), the median free T3 for women was 5.4 pmol/L at 18 to 29, 5.1 at 30 to 39, 5.0 at 40 to 49, 4.9 at 50 to 59 and 4.9 at 60 and over. For men it was 5.6 pmol/L at 18 to 29, 5.6 at 30 to 39, 5.5 at 40 to 49, 5.5 at 50 to 59 and 5.1 at 60 and over.
The lab range is 3.1 to 6.8 pmol/L. A member median is not a target or a diagnosis. If your result is outside the range on your report, talk to your GP.
How to get it: Available as one of our specialty add-on tests. Your test referral is issued straight away when you order. See how a blood test without a referral works.
Add-on test: Thyroid Panel
Related tests: FT3 / FT4 Ratio, Thyroid Stimulating Hormone (TSH), Free Thyroxine (T4), TSH / T4 Ratio
Learn more: What your thyroid results can mean
What your result means: What your hormone results can mean
Guide: TSH and free T4 levels by age, with Vively member data.
The Baseline Health Check reads 70+ markers, then your dietitian turns your results into a plan. $99 a month, first test included.
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