Health Reference LibraryFree T4 blood test results explained: what does your number mean?
This entry is part of the Nutri Tailor Health Reference Library — cited research on supplements, nutrients and adjacent areas of health.
Summary
Free T4 measures the unbound portion of thyroxine, the main hormone the thyroid gland produces, and UK laboratories report the result in picomoles per litre. Unlike many blood markers, the reference range for free T4 genuinely differs between UK laboratories. One NHS laboratory publishes a range of 8 to 18 pmol/L while others use ranges near 12 to 22, and a 2023 study in the European Journal of Endocrinology documented this variation across NHS laboratories directly. The range printed on your own report is therefore the one your result should be read against. The meaning of any free T4 number also depends on the TSH measured alongside, and the recognised patterns are set out below.
How it works
Thyroxine, shortened to T4, is produced by the thyroid gland and circulates mostly bound to carrier proteins. The free T4 test measures the small unbound fraction, which is the portion available to tissues. The pituitary gland adjusts TSH in response to circulating hormone, so TSH and free T4 normally move in opposite directions. This inverse relationship is why the pair is interpreted together and why a result where both move the same way, or where one moves without the other, points to different explanations than a simple underactive or overactive gland.
Safety profile
No supplement corrects overt thyroid dysfunction, and UK guidance manages both underactive and overactive patterns through GP care with prescriptions where indicated. High dose iodine or kelp products can worsen thyroid dysfunction in either direction, and biotin supplements can interfere with thyroid laboratory assays, a point covered in the entry on biotin and blood test interference. Anyone taking levothyroxine should have changes guided by their prescriber, since timing interactions with iron and calcium supplements are well documented.
Practical framework
Read your free T4 against the range printed on your own report, because published UK laboratory ranges differ. Then read the pattern rather than the single number. Raised TSH with low free T4 and suppressed TSH with raised free T4 both belong with a GP. Raised TSH with normal free T4 is the subclinical pattern, and NICE guidance describes repeat testing after three to six months before decisions. Low free T4 with normal TSH most often reflects illness or prescriptions and also warrants GP review. Where thyroid function is normal and symptoms persist, ferritin, vitamin D and B12 are the markers that most often explain the picture.
Sources
- 2023. Variation in the reference range limits of thyroid function tests and association with the prevalence of levothyroxine treatment. European Journal of Endocrinology.
- Adapted Summary of UK Guidelines for the Use of Thyroid Function Tests. UK thyroid function testing guidance (NHS laboratory summary).
- Hypothyroidism and Thyroid Function Tests. NHS Lothian RefHelp (clinical referral guidance).
- Thyroid Function Tests. North Bristol NHS Trust, Severn Pathology.
- Thyroid disease: assessment and management (NG145). National Institute for Health and Care Excellence (NICE).