Free T4pmol/L
Thyroid hormone production
Also known as: FT4, Free Thyroxine
Typical optimal range
12–22 pmol/L
A typical reference range for free T4 is around 12–22 pmol/L, though exact ranges vary between laboratories and analysers, so your result should always be interpreted against your own lab's reference interval. These figures are indicative — the range printed on your own lab report is the definitive one for your results.
Thyroid function test interpretation in UK primary care follows NICE guidance and NHS laboratory standards, including NICE guideline NG145 on thyroid disease identification and management.
What is Free T4?
Free T4 (free thyroxine) is the unbound, biologically active fraction of thyroxine, the main hormone secreted by the thyroid gland. Unlike total T4, it is not attached to carrier proteins in the blood, making it a more accurate reflection of thyroid hormone availability to the body's tissues. Once released, T4 acts largely as a precursor, being converted into the more potent hormone T3 in tissues such as the liver and kidneys. Measuring free T4 alongside TSH gives clinicians a clearer picture of how well the thyroid gland is functioning.
What a high Free T4 means
A raised free T4 can be a signal that the thyroid is producing more hormone than the body needs, a pattern associated with an overactive thyroid (hyperthyroidism), or that levothyroxine replacement is slightly higher than required. It warrants a conversation with your GP rather than immediate alarm.
- Hyperthyroidism. Conditions such as Graves' disease or a toxic multinodular goitre can drive the thyroid to overproduce T4. These are among the most common causes of persistently elevated free T4 in UK clinical practice and are assessed alongside TSH and sometimes thyroid antibodies.
- Excess levothyroxine. People taking levothyroxine for an underactive thyroid may show a raised free T4 if their dose is slightly too high. This is a common and manageable finding that your GP or endocrinologist can address by adjusting the prescription.
- Non-thyroidal illness or medication effects. Certain medicines, including amiodarone and some contrast agents, can interfere with thyroid hormone levels or their measurement. Acute illness can also transiently alter results, which is why a single raised reading is rarely acted upon in isolation.
Free T4 should always be interpreted alongside TSH; a raised free T4 with a suppressed TSH carries different clinical weight than a raised free T4 with a normal TSH, which can sometimes reflect a lab or assay interference.
What a low Free T4 means
A low free T4 alongside a raised TSH is the classic pattern of an underactive thyroid (hypothyroidism), a common and treatable condition in the UK that affects thyroid hormone availability throughout the body.
- Primary hypothyroidism. The most frequent cause is autoimmune thyroiditis (Hashimoto's disease), where the immune system gradually reduces thyroid output. This results in both a low free T4 and a compensatory rise in TSH, and is straightforward to confirm with thyroid antibody testing.
- Insufficient levothyroxine dose. People already on levothyroxine who show a low free T4 may need a dose review; factors such as changes in body weight, concurrent medicines (including some antacids and iron supplements), or altered absorption can all reduce effective levels.
A mildly low free T4 in the context of a normal TSH may reflect a central (pituitary) issue rather than a primary thyroid problem, and is worth flagging to your GP for further assessment.
Should I be worried?
A single free T4 result outside the reference range does not, on its own, indicate a diagnosis. Thyroid function tests can be affected by timing, recent illness, pregnancy, and a range of medicines, so GPs routinely repeat an abnormal result before drawing conclusions. If your free T4 is flagged, your GP will most likely look at it alongside your TSH and may also check thyroid antibodies or request a repeat test after a short interval. Most thyroid conditions, once identified, are highly manageable, so the main value of noticing an out-of-range result is that it opens a productive conversation with your GP at the right time.
What actually helps
While free T4 is primarily regulated by the thyroid gland itself, a few nutritional and lifestyle factors can support healthy thyroid function.
- Adequate iodine intake. Iodine is essential for the synthesis of thyroid hormones including T4. The UK population generally obtains iodine from dairy products and white fish, but those following a vegan diet may be at risk of mild insufficiency; if concerned, a GP can check urinary or serum iodine status.
- Avoid extreme dietary restriction. Very low calorie diets and prolonged fasting can suppress thyroid hormone output. Maintaining a balanced, adequate energy intake supports normal thyroid axis function.
- Moderate consumption of raw goitrogens if iodine intake is low. Foods such as raw cruciferous vegetables (broccoli, kale, cabbage) contain compounds that can mildly interfere with iodine uptake in large quantities, though for most people eating a varied diet this is not a concern. Cooking largely neutralises these compounds.
- Manage stress and sleep. Chronic psychological stress and poor sleep can affect the hypothalamic-pituitary-thyroid axis. While not a primary driver of free T4, supporting overall hormonal balance through rest and stress management is broadly beneficial.
Good supporting evidence points to the following foods as relevant to thyroid hormone production and iodine status.
Dairy (milk, yoghurt)
One of the main dietary sources of iodine in the UK; important for T4 synthesis.
White fish (cod, haddock)
A good source of iodine and selenium, both of which support thyroid hormone production and metabolism.
Eggs
Contain iodine, particularly in the yolk, contributing meaningfully to daily intake for those who eat them regularly.
Seaweed (moderate amounts)
Naturally rich in iodine, though very high or frequent intake can paradoxically suppress thyroid function, so occasional consumption is preferable to daily use.
Brazil nuts (1-2 per day)
An efficient source of selenium, a mineral required for converting T4 to the active T3 form; UK soil selenium levels are relatively low, making dietary sources relevant.
Fortified plant milks (iodine-fortified)
For those not consuming dairy, choosing plant milks fortified with iodine helps maintain adequate intake to support thyroid hormone synthesis.
Track your own Free T4
Upload a lab PDF and Better Days trends Free T4 over time, flags it when it drifts, and suggests foods that help.
More in Thyroid
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.