FT3: what it is, what it tells you and when it’s tested

FT3 is free triiodothyronine: the active form of thyroid hormone, the one that actually makes your cells work. Most of it is made from FT4, which your body converts in the tissues. Doctors read it together with TSH and FT4.

When it’s tested

Doctors mostly order it when they suspect a thyroid that works too hard: if TSH is low, FT3 helps show by how much. Sometimes it is the only hormone that rises, while FT4 stays normal.

For a slow thyroid it matters less: TSH and FT4 count there, and FT3 often stays normal for a long time.

It is not a test for people who feel well. That is why Rosa does not test it. Rosa measures TSH, the first test for the thyroid, and explains it together with the rest of the panel.

What it tells you, and what it does not

If it is high, there is usually too much active hormone in circulation, and the thyroid is working too hard. If it is low, the thyroid is often not the reason: an illness or a period of eating very little brings it down.

On its own it does not make a diagnosis. It is always read with TSH and FT4, and other tests, such as thyroid antibodies, give the cause.

That is why it is read together with other tests:

Normal FT3 levels

The lab decides what is normal, and for FT3 the limits vary quite a lot from one analyser to another. Always look at the limits printed next to your result.

The unit matters too. In pmol/L, many reports put the upper limit between 6 and 7. In pg/mL the numbers are smaller, and the upper limit is usually about 4: 1 pg/mL is about 1.5 pmol/L.

To follow FT3 over time, stay with the same lab if you can: two different analysers can give different numbers on the same blood.

If your FT3 is high or low

What causes high FT3

If FT3 is high and TSH is low, your thyroid is making too much hormone. The most common causes:

Symptoms of high and low FT3

With a high FT3, the symptoms are those of a fast thyroid:

A low FT3 usually causes no symptoms of its own. If you feel tired, the cause is often the illness or diet that brought it down.

Low FT3: what it can mean

If FT3 is low but TSH and FT4 are normal, your thyroid is almost always fine. During a serious illness, a hospital stay, or when you eat very little, your body turns less FT4 into FT3 to save energy. Once you are better, it usually returns to normal.

If FT4 is low too and TSH is high, your thyroid is working too little: this is hypothyroidism, and FT4 matters more. If you take thyroid hormone, your doctor adjusts the dose mainly on TSH.

A low FT3 with a normal TSH is usually not treated with thyroid hormone. The cause is treated.

What can skew the result

A few things change FT3 without any thyroid problem.

When to worry

Talk to your doctor if:

Frequently asked questions

What are normal FT3 levels? It depends on the lab and the unit. In pmol/L, many reports put the upper limit between 6 and 7; in pg/mL about 4. The range on your own report is what counts.

Low FT3 with a normal TSH: should I worry? Usually not. It happens during an illness, after a hospital stay or on a very low-calorie diet, and it is a response of the body, not a thyroid problem. If it stays low once you are well, talk to your doctor.

High FT3 with a normal FT4: what does it mean? With a low TSH, it can be a thyroid starting to work too hard, or a nodule that makes mostly FT3. With a normal TSH, the test method or a medicine is often behind it. Your doctor usually repeats it.

Do I need FT3 if I take levothyroxine? Usually not. The dose is adjusted mainly on TSH. Your doctor decides whether anything else is needed.

Does Rosa test FT3? No. Rosa measures TSH, the first test for the thyroid, and high-sensitivity CRP. If your doctor wants FT3, they order it separately.

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