Free T4
Free Thyroxine
What this test measures
Free T4 measures the active thyroid hormone in your blood. Your thyroid gland makes T4, which controls your metabolism, energy levels, and many body functions. This test is usually done alongside TSH.
Understanding the numbers
Typical range: 9-22 pmol/L
Typical UK range. Labs vary (some use 9-25 pmol/L). Always check your own report. Results are best interpreted alongside TSH.
- Low (underactive): 0–9 pmol/L
- Normal: 9–22 pmol/L
- High (overactive): 22–40 pmol/L
Key numbers
- 9 pmol/L — Lower limit — low suggests underactive
- 22 pmol/L — Upper limit — high suggests overactive
Reference range is a typical UK laboratory range; labs use slightly different limits. Free T4 is always interpreted together with TSH — the pattern of the two tells you more than either alone (e.g. low TSH + high T4 = overactive thyroid).
Ranges can vary slightly between laboratories — your report may show slightly different numbers. This page does not read or interpret your result; your GP looks at it alongside your history and other tests.
What a higher-than-normal result can mean
- Your thyroid may be overactive (hyperthyroidism)
- Symptoms can include weight loss, anxiety, and feeling hot
- Can be caused by conditions like Graves' disease
- Treatment is available to bring levels back to normal
Questions you could ask your GP
- Do I need thyroid medication?
- How often should my thyroid levels be monitored?
- What symptoms should I watch out for?
- Should I be referred to an endocrinologist?
Example stories
These are made-up examples to show how results are talked about in real appointments. They are not real patients.
Well on levothyroxine
Nisha has hypothyroidism on levothyroxine 100 micrograms. Her annual check shows TSH 1.8 and free T4 15 pmol/L.
What the GP said: “Everything looks stable. Keep on the same dose and we'll check again in a year. No change needed.”
A free T4 in the upper half of normal with a low-normal TSH is the usual picture on a well-controlled levothyroxine dose. It's the pattern that matters, not just one number.
Low T4, raised TSH — new hypothyroidism
Kavita's free T4 was 7 pmol/L and her TSH was 28. She had been tired, gaining weight, and feeling cold.
What the GP said: “Your thyroid is clearly underactive — the low T4 and high TSH confirm it. I'll start you on levothyroxine and check your thyroid levels again in 6-8 weeks.”
What happened next: Kavita's dose was built up from 50 to 100 micrograms over three months. At that point her T4 was 17 and her TSH was 2.1, and she felt much better.
Low T4 with high TSH is the classic pattern for primary hypothyroidism. Treatment is usually straightforward and symptoms improve within weeks to a few months.
Racing heart and tremor
Amina's free T4 was 34 pmol/L and her TSH was undetectable. She had a fine hand tremor and a resting pulse of 108.
What the GP said: “This is a very overactive thyroid. We'll start carbimazole and propranolol today and I'm referring you urgently to endocrinology. You'll feel the heart symptoms settle within a few days.”
What happened next: Amina was diagnosed with Graves' disease. After 18 months of carbimazole she was in remission and off treatment.
A clearly raised T4 with a suppressed TSH is hyperthyroidism. It is almost always treatable. The symptoms (racing heart, weight loss, shakiness) ease quickly once treatment starts.
Low T4 but normal TSH — sick euthyroid
George was admitted with pneumonia. His free T4 was 8 pmol/L but his TSH was only 1.2 — not raised as you would expect in true hypothyroidism.
What the GP said: “This is the 'sick euthyroid' pattern — severe illness can drop the thyroid numbers without meaning there is thyroid disease. We won't start treatment; we'll repeat the bloods once you're recovered.”
What happened next: Six weeks after discharge George's free T4 was 13 and his TSH was 1.4. No treatment was needed.
A low T4 with a normal (not raised) TSH in someone acutely unwell is often non-thyroidal — the body slows thyroid signalling during severe illness. The right action is to wait and repeat after recovery.
Before you act on anything here
This page is general information, written to help you understand — it is not personal medical advice and it cannot tell you what is right for you. Talk to your own GP, pharmacist or nurse about your own health.
- Need help now but it's not an emergency? Call NHS 111 or use 111.nhs.uk.
- Emergency? Call 999 or go to A&E.