Home › Biomarkers › Free T3
This level shows how much active thyroid hormone is circulating freely in your blood. A high level can point to a thyroid that makes too much hormone, a low level to a thyroid that makes too little.[1]
On this page you can read what Free T3 is, what the guidelines say about a level that is too low or too high, what affects your level and which levels are best to look at alongside it.
Also known as: free triiodothyronine, free T3. At the lab: FT3.
Free T3 is the active part of the thyroid hormone T3 in your blood. Your thyroid is a small, butterfly-shaped gland at the bottom of your throat. It makes two hormones: T3 and T4. Free T3 is the form that can enter your tissues, right where it is needed.[1]
Together, thyroid hormones control how your body uses energy. They affect your weight, your heart, your body temperature, your muscle strength and even your mood. Most of your T3 is bound to certain proteins and cannot enter your tissues. A Free T3 test measures only the free part.[1]
Free T3 is measured in blood taken from a vein in your arm. The doctor usually looks at the result together with your TSH and T4. A high value can point to a thyroid that makes too much hormone, and a low value to one that makes too little. A normal value on its own does not rule out a thyroid condition.[1]
A T3 test mainly helps to check whether your thyroid is making too much hormone. The test can also help find what is causing it.[1]
The pituitary gland is a pea-sized gland at the base of your brain. It makes TSH, the hormone that tells your thyroid how much T3 and T4 to make. That is why an abnormal T3 level can also point to a problem with the pituitary gland.[1]
Your doctor may ask for a T3 test if you have symptoms that fit too much thyroid hormone. Think of restlessness, shaky hands, heavy sweating, poor sleep, tiredness or an irregular heartbeat.[1]
Labplusarts, the lab BioProfile works with, uses a reference range of 3.5 to 5.9 pmol/l for Free T3. The lab calls a level between those limits normal.[2]
| Free T3 | Can point to | Alongside which other results |
|---|---|---|
| High[1] | A thyroid that makes too much hormone | In general |
| High[1] | A problem in the thyroid itself that leads to too much hormone | With a low TSH and a high Free T4 |
| High[1] | A pituitary condition that leads to too much thyroid hormone | With a normal or high TSH and a high Free T4 |
| Low[1] | A thyroid that makes too little hormone | In general |
| Low or normal[1] | A problem in the thyroid itself that leads to too little hormone | With a high TSH and a low Free T4 |
| Low or normal[1] | A pituitary condition that leads to too little thyroid hormone | With a low TSH and a low Free T4 |
Source: MedlinePlus, U.S. National Library of Medicine. Patterns for T3, free or total, together with TSH and Free T4, without fixed numbers.[1]
A low Free T3 can point to a thyroid that makes too little hormone. Even so, a T3 test is usually not the first choice for this. Other thyroid tests show it sooner.[1]
A high Free T3 can point to a thyroid that makes too much hormone. If your TSH or T4 result is abnormal or unclear, your doctor can use a T3 test to confirm or rule this out. Ask your doctor what your combination of results means.[1]
BioProfile is working on its own zones for each value, from risk to optimal. A doctor reviews those zones before they appear in your result.
Certain medicines and supplements can affect your result. Your doctor will let you know if you need to stop taking anything before the test. Do not stop taking a medicine on your own unless your doctor tells you to.[1]
If you take thyroid hormone as a medicine, a T3 test can show whether your dose is keeping your hormone levels within a healthy range.[1]
How much T3 your thyroid makes depends on TSH from your pituitary gland. That way, a pituitary condition can lead to too much or too little thyroid hormone.[1]
Free T3 is included in BioProfile 121 and 150.[3]
The BioProfile editorial team wrote this page on 10 October 2026 from the sources below, which we read on 10 October 2026. Every number on this page appears in one of those sources, with that source's number next to it. An expert has yet to check this page; after that check, the expert's name and the date of the check will appear at the top. This page offers explanation, not a diagnosis. Discuss a value outside the range with your GP.
