FT3
Note: this site is for informational purposes only. To view test results or book a test, use the NHS app in England or contact your GP.
The FT3 test measures the level of free triiodothyronine in your blood. It is mainly used to help diagnose an overactive thyroid (hyperthyroidism) and to monitor treatment. Your doctor will usually request it alongside other thyroid tests.
Who needs this test
Your GP or specialist may request an FT3 test if you have an abnormal TSH (thyroid-stimulating hormone) or FT4 (free thyroxine) result.
It is most useful when your doctor suspects you have an overactive thyroid. You may have symptoms such as unexpected weight loss, a fast heartbeat, feeling anxious or shaky, or difficulty sleeping.
FT3 is not usually requested to investigate an underactive thyroid (hypothyroidism). TSH and FT4 are the main tests for that.
Preparing for your test
You do not need to do anything special before this test.
However, some medicines can affect your FT3 result. Tell your doctor about everything you are taking. This includes prescribed medicines, over-the-counter medicines, and supplements.
Understanding your results
What the test measures
Triiodothyronine (T3) is one of two main hormones made by the thyroid gland. The other is thyroxine (T4). The thyroid gland sits in your neck, in front of your windpipe.
Although T3 makes up less than 10% of thyroid hormone in the blood, it is around four times more powerful than T4. It is thought to cause most of the effects of thyroid hormones on the body, including controlling your metabolism — the process by which your body uses energy.
Most T3 in your blood is attached to a protein and is not active. A small portion is unattached and free to act in your body. This unattached portion is what the FT3 test measures. Most laboratories measure and report FT3 as standard.
FT3 results are always interpreted alongside TSH and FT4. A single result does not tell the full story. Your doctor will consider your symptoms, your medical history, and any medicines you are taking before reaching any conclusions.
What your results mean
If your FT3 is high
A high FT3 level suggests your thyroid gland may be overactive. This is called hyperthyroidism.
FT3 can rise earlier than FT4 when the thyroid becomes overactive, and it may stay raised for longer when treatment begins. This makes FT3 particularly useful for diagnosing and monitoring hyperthyroidism.
Common causes of a high FT3 include:
- Graves’ disease — an autoimmune condition and the most common cause of hyperthyroidism
- Toxic nodular goitre — overactive nodules within the thyroid gland
- Thyroiditis — inflammation of the thyroid gland, which can temporarily release excess hormones
- Taking too much thyroid hormone medication
If your FT3 is low
A low FT3 level can suggest an underactive thyroid (hypothyroidism). However, FT3 is not usually the test your doctor will use to investigate this — TSH and FT4 are more reliable for that purpose.
A low FT3 is also common in people who are seriously unwell for other reasons. When you are very ill, your body reduces the conversion of T4 into T3. Most people admitted to hospital with a serious illness will have a low FT3. This is known as non-thyroidal illness or sick euthyroid syndrome. It does not usually mean there is a thyroid problem.
Because of this, FT3 is not used as a routine thyroid test for people in hospital.
Questions to ask your doctor
What happens next
If your FT3 result is abnormal, your doctor will look at it together with your TSH and FT4 results. They will consider your symptoms and any relevant medical history.
If hyperthyroidism is confirmed, treatment options include tablets that reduce thyroid hormone production, radioiodine treatment, or surgery. Your doctor will work with you to find the right approach.
If hypothyroidism is suspected, TSH and FT4 are the tests that will guide treatment. Most people with hypothyroidism are treated with levothyroxine tablets, which replace the missing thyroid hormone.
What can affect your results
Several things can affect your FT3 result:
- Serious illness — being very unwell lowers FT3 even when there is no thyroid problem. This is why FT3 is not used as a routine test in hospitals.
- Some medicines — large doses of aspirin and certain other drugs can affect total T3 results, though FT3 is usually less affected than total T3. Always tell your doctor what you are taking.
- Pregnancy — total T3 levels can rise during pregnancy, but FT3 levels do not usually change significantly. A raised total T3 in pregnancy does not mean you have thyroid disease.
Always tell your GP or specialist about all medicines, supplements, and vitamins you are taking before the test.
Other tests you might need
FT3 is always used alongside other thyroid tests. Your doctor may also request:
- TSH (thyroid-stimulating hormone) — the first-line test for thyroid problems
- FT4 (free thyroxine) — usually tested alongside TSH and FT3
- Thyroid antibodies — to check for autoimmune thyroid conditions such as Graves’ disease or Hashimoto thyroiditis
About thyroid disease
The thyroid gland produces hormones that control many body functions. When it produces too much or too little hormone, it can affect your energy, weight, mood, and heart rate. To learn more, visit the NHS pages on overactive thyroid and underactive thyroid.