Free T3
Free Triiodothyronine
What this test measures
Free T3 is the most active form of thyroid hormone. Your body converts T4 into T3. This test is usually only done if your TSH or T4 results are abnormal and your GP needs more information.
Understanding the numbers
Typical range: 3.5-6.5 pmol/L
Typical UK range. Some labs use wider ranges (up to 7.8 pmol/L). Always check your own report. T3 is mainly useful when investigating an overactive thyroid.
- Low: 0–3.5 pmol/L
- Normal: 3.5–6.5 pmol/L
- High (overactive): 6.5–12 pmol/L
Key numbers
- 3.5 pmol/L — Lower limit
- 6.5 pmol/L — Upper limit
Reference range is a typical UK laboratory range; labs differ. Free T3 is mainly used to confirm overactive thyroid — particularly when T4 is normal but TSH is suppressed ('T3 toxicosis'). It is not usually done as a first-line thyroid test.
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
- Can cause symptoms like rapid heartbeat and weight loss
- Sometimes raised when T4 is normal (T3 toxicosis)
- Treatment can help bring levels back to normal
Questions you could ask your GP
- Why was my T3 level tested?
- How does this result fit with my other thyroid tests?
- Do I need any treatment?
- When should this be rechecked?
Example stories
These are made-up examples to show how results are talked about in real appointments. They are not real patients.
T3 toxicosis — suppressed TSH, normal T4
Priya's TSH was suppressed at under 0.01 but her free T4 was 18 — top end of normal. The lab added a free T3, which came back at 9.2.
What the GP said: “This is 'T3 toxicosis' — your thyroid is overactive but it's showing in the T3 rather than T4. It is a real form of hyperthyroidism and needs treatment. I'll refer you to endocrinology and start carbimazole.”
What happened next: Priya was diagnosed with a toxic nodule on her thyroid. She had radioiodine treatment and her hormone levels returned to normal.
A suppressed TSH with a normal T4 isn't always 'subclinical' — sometimes the T3 is raised and causing symptoms. This is when free T3 earns its place as a second-line test.
Monitoring treatment
Rahul has Graves' disease, on carbimazole for the last year. His TSH, T4 and T3 were all in the normal range at the last three clinic visits.
What the GP said: “Your thyroid numbers have been stable for a year. We can start planning a gentle withdrawal of the carbimazole over the next few months and see if the Graves' stays in remission.”
What happened next: Rahul came off carbimazole and his thyroid function stayed normal at six and twelve months.
Stable free T3, T4 and TSH over many months in Graves' disease is a sign that remission may be achievable. About half of people with Graves' can come off tablets long-term.
Low T3 during hospital admission
Harold was admitted with heart failure. His free T3 was 2.6 pmol/L, his T4 was at the lower end of normal, and his TSH was 2.1.
What the GP said: “This is the 'low T3' pattern we often see in severe illness — the body dampens down T3 during acute stress. It isn't thyroid disease on its own, and starting thyroid tablets would not help.”
What happened next: Six weeks after discharge Harold's free T3 had risen to 4.1 without any thyroid treatment.
A low T3 in a severely unwell person is almost always non-thyroidal illness. Treating it with thyroid hormone can do more harm than good — the right answer is to treat the underlying illness.
Feels hyper on levothyroxine
Meera takes levothyroxine for hypothyroidism. Her TSH was low-normal at 0.3 and she felt jittery and wasn't sleeping well. Her free T3 and T4 were in the upper part of the normal range.
What the GP said: “Your numbers are technically normal but your symptoms suggest your dose is a touch too high. Let's drop from 125 to 112 micrograms and recheck in eight weeks.”
What happened next: After dose reduction Meera's TSH came up to 1.6 and her jittery feeling settled.
Symptoms matter, not just numbers. Some people on levothyroxine feel over-replaced even when the blood tests look 'normal'. Careful dose titration based on how you feel, alongside the bloods, is part of good thyroid care.
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.