Free T3pmol/L
Active thyroid hormone
Also known as: FT3, Free Triiodothyronine
Typical optimal range
3.1–6.8 pmol/L
A typical optimal range for Free T3 is around 3.1–6.8 pmol/L, though reference intervals vary between laboratories depending on the assay used, so your result should always be interpreted against your own lab's reference range. These figures are indicative — the range printed on your own lab report is the definitive one for your results.
Thyroid function testing and the management of thyroid disorders in the UK follow NICE guidance and NHS laboratory standards; the British Thyroid Association also publishes clinical statements on thyroid testing that inform UK practice.
What is Free T3?
Free T3 (triiodothyronine) is the biologically active form of thyroid hormone — the small fraction circulating in the blood that is not bound to carrier proteins and is therefore available to enter cells and regulate metabolism, energy production, heart rate, and temperature. Most T3 in the body is produced not by the thyroid gland directly, but by conversion of thyroxine (T4) in peripheral tissues such as the liver and kidneys. Measuring the free fraction, rather than total T3, gives a more accurate picture of how much active hormone is actually available to the body, and can help clarify the cause of thyroid symptoms when TSH or Free T4 results are ambiguous.
What a high Free T3 means
A Free T3 result above the reference range is a signal that warrants follow-up with a GP — it may indicate that the thyroid is overactive or that thyroid hormone levels are being influenced by another factor, but it does not confirm a diagnosis on its own.
- Hyperthyroidism. Conditions such as Graves' disease or a toxic nodular goitre can cause the thyroid to produce excess hormone, raising both Free T4 and Free T3. This is the most common clinical reason for an elevated Free T3 and usually requires investigation and management by a GP or endocrinologist.
- T3-containing medication. People taking liothyronine (T3 tablets) or desiccated thyroid preparations may show a raised Free T3, particularly if blood is drawn close to the time of their dose. This is worth flagging to your GP when reviewing results.
- Acute illness or thyroiditis. Inflammation of the thyroid gland (thyroiditis) in its early phase can transiently release stored hormone into the bloodstream, temporarily elevating Free T3 and Free T4 before levels may later fall.
Free T3 is not routinely requested as a first-line thyroid test by the NHS; it is most meaningful when interpreted alongside TSH and Free T4. A single mildly raised result may need repeating before any clinical decision is made.
What a low Free T3 means
A Free T3 below the reference range can reflect reduced thyroid hormone production or impaired conversion of T4 to T3, and is worth discussing with a GP, particularly if you have symptoms such as fatigue, feeling cold, or unexplained weight gain.
- Hypothyroidism or poor T4-to-T3 conversion. An underactive thyroid, or conditions that impair the enzymes (deiodinases) that convert T4 into active T3 — such as chronic illness, significant nutrient deficiencies, or liver problems — can result in a low Free T3 even when TSH appears only mildly abnormal.
- Low energy intake, low carbohydrate diets, or overtraining. Prolonged calorie restriction, very low carbohydrate diets, and excessive exercise without adequate recovery can reduce peripheral T4-to-T3 conversion as an adaptive response, lowering Free T3 without necessarily indicating thyroid disease. This is sometimes called 'low T3 syndrome' and the evidence suggests it often reverses with dietary normalisation.
A low Free T3 in the context of a normal TSH and Free T4 can be difficult to interpret and the clinical significance is debated; your GP will consider the full picture including your symptoms and other results.
Should I be worried?
A single out-of-range Free T3 result is a prompt to have a conversation with your GP, not a cause for immediate alarm — thyroid hormone levels can fluctuate with illness, stress, diet, and the timing of any thyroid medication. Your GP will usually want to review your TSH and Free T4 alongside Free T3, as these markers are most informative when considered together. If your result is persistently abnormal, or if you have symptoms that concern you, your GP may arrange repeat testing, check for thyroid antibodies, or refer you to an endocrinologist. The NHS recommends that thyroid function tests are interpreted in clinical context, so bring any symptoms — however vague — to your appointment.
What actually helps
Certain lifestyle and dietary factors can support healthy thyroid hormone production and the conversion of T4 to active T3.
- Adequate carbohydrate intake. Very low carbohydrate and very low calorie diets are associated with reduced T4-to-T3 conversion as an adaptive response. Including sufficient carbohydrates — particularly around exercise — can help maintain normal conversion in otherwise healthy people.
- Selenium. Selenium is essential for the deiodinase enzymes that convert T4 to T3 in peripheral tissues. The UK diet can be low in selenium, particularly where food is grown on selenium-poor soils; Brazil nuts, fish, and eggs are practical dietary sources.
- Zinc. Zinc plays a role in thyroid hormone production and receptor binding. Deficiency is associated with impaired thyroid function; a varied diet including meat, shellfish, legumes, and seeds generally provides adequate zinc for most adults.
- Stress management. Chronic psychological stress and elevated cortisol can suppress thyroid hormone conversion. Regular rest, moderate exercise, and evidence-based stress reduction approaches such as mindfulness are reasonable supportive measures.
The following foods have good supporting evidence for maintaining the nutrients most relevant to thyroid hormone production and conversion.
Brazil nuts
One of the richest dietary sources of selenium; one to two nuts per day can meaningfully contribute to selenium intake without risking excess.
Oily fish (e.g. salmon, mackerel, sardines)
Provide selenium and iodine, both of which are required for thyroid hormone synthesis and conversion.
Eggs
A practical everyday source of both selenium and iodine, two key nutrients for thyroid function.
Shellfish (e.g. oysters, mussels)
Among the best dietary sources of zinc, which supports thyroid hormone production and receptor activity.
Wholegrains and legumes
Provide zinc and complex carbohydrates that help sustain adequate energy availability, supporting normal T4-to-T3 conversion.
Dairy products
A significant source of iodine in the UK diet, which is required by the thyroid gland to synthesise T4 and T3.
Track your own Free T3
Upload a lab PDF and Better Days trends Free T3 over time, flags it when it drifts, and suggests foods that help.
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Sources & further reading
These are reputable UK health resources for further reading. This page is written to be consistent with their guidance but is not a substitute for them.
For information only — not medical advice, and not individually reviewed by a clinician. Reference ranges vary by lab and individual. Discuss any results with a qualified clinician before making changes.