Free T3 test: normal range and what high or low results mean

A free T3 test measures free triiodothyronine (FT3), the unbound share of the thyroid hormone that acts on the body's cells. It adds most when TSH is low and the question is whether the thyroid is overactive. It adds less when the question is an underactive thyroid, where TSH and free T4 usually change sooner.

What free T3 measures

The thyroid releases mostly thyroxine (T4). To act, T4 is converted into triiodothyronine (T3) by the removal of one iodine atom, mainly in the liver and in tissues where T3 works, such as the brain. Like T4, T3 travels almost entirely bound to proteins: about 0.3% of it is free, and the free fraction is the active form.

Laboratories can measure total T3 or free T3, and not every reference source favours the free version. The American Thyroid Association describes free T3 measurement as often not reliable, and Mayo Clinic Laboratories notes that total T3 usually suffices, with free T3 most useful when carrier proteins are altered, as in pregnancy. That is worth keeping in mind when a free T3 result disagrees with TSH and free T4.

Normal range

Free T3 is reported in pmol/L (SI units) or pg/mL. To convert pg/mL to pmol/L, multiply by 1.54. Published adult ranges from three reference laboratories:

Children have higher ranges, pregnancy shifts free T3, and each laboratory sets its range for its own method, so read your result against the range printed on your own report.

What a high free T3 usually points to

A high free T3 with a low TSH usually points to an overactive thyroid (hyperthyroidism). In most cases free T4 is high too, but in a small group T3 rises while free T4 stays normal, a pattern called T3 toxicosis. This is the main reason free T3 is measured when TSH is low. A higher dose of thyroid hormone medicine than needed can also raise it. A high free T3 alongside a high free T4 and a TSH that is normal or high is unusual, and can point to the pituitary gland.

With a high result, a doctor typically reads it against TSH and free T4 from the same sample and may check TSH receptor antibodies, which are linked with Graves' disease.

What a low free T3 usually points to

A low free T3 can reflect an underactive thyroid, but T3 tends to be the last of the three to fall, so it is rarely needed to find one: people can have a high TSH and a low free T4 with a normal T3. More often, a low free T3 on its own reflects the body's response to illness or to eating too little. Changes in T3 can appear within 24 hours of an illness and are also seen with poor nutrition and starvation, and they usually settle with recovery.

When TSH and free T4 are both normal, a thyroid condition is unlikely. Doctors are advised to rule out illness and medicines as explanations before any further thyroid testing.

What moves free T3

• Illness and food intake. Illness, starvation and recovery shift T3 more than the other thyroid markers.
• Pregnancy. Pregnancy changes carrier proteins and hormone levels, so laboratories use separate ranges for each trimester.
• Medicines. Amiodarone can change TSH, free T4 and free T3. Heparin can raise measured free T4 and free T3, and so can too high a dose of thyroid hormone medicine.
• Biotin. In a 2017 study, a week of high-dose biotin shifted results on some free T3 assays. Newer versions of some assays are designed to tolerate biotin, so the effect depends on the laboratory's method.

Preparing for the test

A free T3 test needs no special preparation. Mention every medicine and supplement you take, including biotin and thyroid medicine, and do not stop a prescribed medicine unless your doctor tells you to. Because illness and very low food intake lower T3, a result taken during an acute illness or a period of very restricted eating is harder to read.

How Clare's panel handles it

In Clare's panel, Free T3 (FT3) is measured by the lab, in the same blood draw as Thyroid-Stimulating Hormone (TSH) and Free T4 (FT4). It is read with those two rather than on its own.

Why it matters in the UAE

Ramadan can shift free T3 in people who take levothyroxine. In 481 patients at a UAE endocrine centre, free T3 and free T4 fell and TSH rose one to two weeks after Ramadan, and all three were back in the normal range 3 to 6 months later. A free T3 taken soon after Ramadan is read with that in mind.

Price in the UAE

Free T3 is usually sold within a thyroid function test. When we read public prices on 11 October 2026, a thyroid function test (usually TSH, free T4 and free T3) cost from 99 to 300 at 6 UAE providers. VAT was rarely stated, some prices were offers, and home collection can add a fee.

Related tests

Free T3 belongs with TSH and Free T4 in the thyroid test. For how we read the three together, read How we read a thyroid panel: TSH, free T4 and free T3.

Why the number means more over time

Free T3 moves with illness and eating more than the other thyroid markers, so one low or high value is weak evidence by itself. A repeat result after recovery, read with TSH and free T4, says much more. For adults with untreated subclinical hyperthyroidism, NICE suggests measuring TSH every 6 months and adding free T4 and free T3 when TSH is outside the range. Clare is opening with a waitlist at clare.health.