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Free T4 (only if TSH is abnormal)

Free T4 is the active, unbound form of thyroid hormone that regulates your metabolism and is usually measured when your TSH is abnormal.

What is Free T4 (only if TSH is abnormal)?

Free T4 is the active, unbound form of the thyroid hormone thyroxine. Only this free fraction enters your cells and regulates your metabolism, energy, and body temperature. Most T4 in your blood is locked to transport proteins and is not usable at that moment. That is why free T4 is more reliable than total T4: the total value can shift with pregnancy or the contraceptive pill without your thyroid actually behaving differently. Free T4 is almost always read together with TSH — and usually only measured once TSH is abnormal.

Why is Free T4 (only if TSH is abnormal) relevant?

TSH reflects what your brain thinks the thyroid should do; free T4 reflects what the thyroid is actually producing. Together they reveal where a problem sits and how significant it is — exactly why free T4 is added once TSH is abnormal. The classic pattern of an underactive thyroid: a high TSH with a low or low-normal free T4. A high TSH with a still-normal free T4 means the thyroid is still compensating. For people already on thyroid medication (levothyroxine), free T4 alongside TSH is the key value for judging whether the dose is right.

Free T4 (only if TSH is abnormal) high or low — what it means

Almost never read free T4 on its own — it belongs with TSH. TSH is the more sensitive marker for early changes; free T4 adds the context to determine the severity and direction once TSH is abnormal. Test in the morning where possible for comparable results. Biotin supplements (common in hair and nail products) can interfere with the measurement and produce a false picture of an overactive thyroid — pause them 2–3 days before testing. After a change in thyroid medication, re-test only after 6–8 weeks, because the system needs that time to reach a new steady state.

Free T4 (only if TSH is abnormal) reference ranges

Normal (adults)Strongly dependent on age, sex and assay method; always use your own lab's reference interval.10-25 pmol/L

Cut-offs for free T4 differ considerably between labs and assay methods (lower limit roughly 9-14, upper limit 19-24 pmol/L). Never read free T4 on its own — always together with TSH. Use your own laboratory's reference interval.

Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.

Read about our scientific approach →Read the guide: Hormones →

Frequently asked questions

What is a normal free T4 level?

In adults, free T4 (FT4) usually sits between 10 and 25 pmol/L. This range depends strongly on age, sex and the assay used, so the exact lower limit (roughly 9-14) and upper limit (19-24) vary by lab. Always follow your own laboratory's reference interval, and never read free T4 on its own — always together with TSH.

What does a low free T4 mean?

A low free T4 (below your lab's lower limit, often around 10 pmol/L) fits with an underactive thyroid (hypothyroidism). The classic pattern is a low free T4 together with an elevated TSH. Free T4 alone is not enough for a diagnosis — it is always assessed alongside TSH.

What does a high free T4 mean?

A high free T4 (above your lab's upper limit, often around 25 pmol/L) fits with an overactive thyroid (hyperthyroidism), usually with a low TSH. Note: high-dose biotin supplements (common in hair and nail products) can interfere with some assays and produce a false picture of an overactive thyroid; pause them 2-3 days before testing.

When is an abnormal free T4 a concern?

A free T4 outside the reference interval (roughly below 10 or above 25 pmol/L) together with an abnormal TSH points to a genuine thyroid disorder that may need treatment. A normal free T4 with an abnormal TSH is called 'subclinical' — a grey zone that warrants attention but is not always treated right away. Always discuss abnormal results with your doctor.

What should I do about an abnormal free T4 result?

Never judge free T4 on its own — assess it together with TSH, and test in the morning where possible for comparable results. On thyroid medication (levothyroxine)? Then free T4 alongside TSH is the key value for judging whether the dose is right — re-test only 6-8 weeks after a change, because the system needs that time to reach a new steady state. Discuss abnormal values with your doctor.

Related biomarkers

TSH
TSH is the hormone your pituitary uses to steer your thyroid and the most sensitive screen of your thyroid function.

Free T4 (only if TSH is abnormal) is one of the biomarkers in the Optimize blood test. Book a blood draw at any of 238+ partner labs in the Netherlands, or upload your existing results in the app.

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