Free T4 test: normal range and what high or low results mean

A free T4 test measures free thyroxine (FT4), the share of the thyroid's main hormone that is not bound to proteins in the blood and can reach the body's tissues. Read with TSH, it shows whether the thyroid is making too little or too much hormone, and whether the pituitary gland is responding as expected.

What free T4 measures

Thyroxine, or T4, is the main hormone the thyroid makes. Almost all of it travels bound to carrier proteins, and the small free fraction is the part tissues can use. The liver and other tissues, including the brain, then convert T4 into T3, the form that acts on cells.

Laboratories can measure total T4, which counts bound and free hormone together, or free T4. Total T4 rises whenever carrier proteins rise, as happens in pregnancy and with oestrogen, including the contraceptive pill. Free T4 is far less affected, which is why it is the measure normally used alongside TSH.

Normal range

Free T4 is reported in pmol/L (SI units) or ng/dL. To convert ng/dL to pmol/L, multiply by 12.87. Published adult ranges from three reference laboratories:

Two NHS laboratories publish quite different ranges, a reminder that the range belongs to the laboratory and its method. Children have their own ranges, and in pregnancy laboratories use separate ranges for each trimester. Read your result against the range printed on your own report.

What a low free T4 usually points to

• Low free T4 with high TSH is the usual pattern of an underactive thyroid (primary hypothyroidism). The common cause is autoimmune thyroiditis (Hashimoto's disease); others include thyroid surgery and radioactive iodine treatment.
• Low free T4 with low or normal TSH is less common. It can follow a serious illness and settle with recovery, or point to a problem with the pituitary gland. When there is no clear explanation, doctors usually check the other pituitary hormones.

What a high free T4 usually points to

• High free T4 with low TSH is the usual pattern of an overactive thyroid (hyperthyroidism). Common causes include Graves' disease, overactive thyroid nodules and some stages of thyroiditis. Too much iodine, and a higher dose of thyroid hormone medicine than needed, can do the same.
• High free T4 with normal or high TSH is unusual. It is most often explained by a medicine such as amiodarone or heparin, by thyroid hormone treatment, or by interference with the assay, and rarely by a pituitary or inherited condition.

With a free T4 outside the range, a doctor typically reads it against TSH from the same sample, adds free T3 when TSH is low, and may check thyroid antibodies to look for an autoimmune cause.

What moves free T4

• Illness. Serious illness can shift thyroid hormone results for a while without thyroid disease, and Mayo Clinic Laboratories notes that free T4 can be misleading in people with major illness.
• Pregnancy and oestrogen. These raise total T4 through carrier proteins. Free T4 is less affected but still has its own ranges in pregnancy.
• Medicines. Heparin, even at low doses, can raise measured free T4, and high doses of furosemide can do the same by displacing T4 from its carrier proteins. Amiodarone and lithium can change thyroid function itself.
• Biotin. Free T4 is measured with competitive immunoassays, and high-dose biotin can push some of them falsely high. Mayo Clinic Laboratories, for example, asks people not to take biotin-containing supplements for 12 hours before its free T4 test.
• Sample handling. In people receiving heparin, free T4 can keep rising in the tube, so laboratories aim to analyse such samples without delay.

Preparing for the test

In most cases a free T4 test needs no special preparation. If it is drawn with tests that need fasting, follow those instructions. Tell the person ordering the test about every medicine and supplement you take, especially biotin, thyroid medicine and heparin, and keep taking prescribed medicines unless your doctor tells you otherwise.

How Clare's panel handles it

In Clare's panel, Free T4 (FT4) is measured by the lab, in the same blood draw as Thyroid-Stimulating Hormone (TSH) and Free T3 (FT3), so it is always read beside the signal that controls it.

Why it matters in the UAE

For people who take levothyroxine and fast in Ramadan, results can shift for a few weeks. In 481 patients at a UAE endocrine centre, free T4 and free T3 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 T4 taken soon after Ramadan is read with that in mind.

Price in the UAE

Free T4 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 T4 is read with TSH and Free T3; together they make up 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 T4 stays fairly steady within each person, while population ranges are wide, so a value can sit inside the range and still be a real change for that person. A run of results shows that drift. NICE suggests measuring free T4 as well as TSH for adults whose symptoms continue after starting thyroid medicine, and checking TSH and free T4 once a year for adults with untreated subclinical hypothyroidism and signs of underlying thyroid disease, or every 2 to 3 years without them. Clare is opening with a waitlist at clare.health.