Thyroid
Free T4 (Thyroxine)
Thyroid
Main thyroid hormone. Used alongside TSH for complete thyroid assessment.
VAT-exempt medical service
Thyroxine (T4) is the main hormone secreted by the thyroid gland. In the bloodstream, most T4 is bound to carrier proteins; the unbound portion, Free T4 (FT4), is the biologically active fraction available for conversion to the more potent T3. Measuring FT4 is considered one of the most important thyroid tests, and it is best interpreted alongside TSH and Free T3 for a complete assessment.
A Free T4 test is a core component of any thyroid evaluation. It helps identify whether the thyroid is producing adequate hormone and is essential for diagnosing hypothyroidism and hyperthyroidism. Regular monitoring is particularly important if you are on thyroid medication, have thyroid-related symptoms, or are pregnant, since thyroid demands increase during pregnancy.
What a high value can mean
Elevated Free T4 suggests the thyroid is overproducing hormone (hyperthyroidism). Graves' disease is the most common autoimmune cause. Other causes include overactive thyroid nodules, excessive iodine intake, thyroiditis, and taking too much thyroid replacement medication. In hyperthyroidism, TSH is typically low while T4 and T3 are normal to high.
What a low value can mean
Low Free T4 indicates that the thyroid is underproducing hormone (hypothyroidism). The most frequent cause is Hashimoto's thyroiditis. Other contributing factors include iodine deficiency, chronic inflammation, calorie restriction, low oestrogen or testosterone, and deficiencies in selenium, zinc, iron, and B vitamins. Very low-carbohydrate diets may also suppress T4 production.
No advance preparation is needed. Do not take thyroid medication until after the test. Avoid vigorous exercise on the morning of the test. Test before 10 am for best consistency. Bring valid identification. If you take a supplement containing biotin (often sold for hair and nails), pause it for two days before the test, as high doses can distort the result.
Order your test
Choose your test and pay online. Your lab referral is created straight away.
Give your sample
Visit any of our partner sampling locations. No appointment needed at most of them.
The lab analyses it
Your sample is analysed at an accredited Swedish laboratory.
Get your results
Your values appear in your account with reference ranges and an explanation of each marker.
Total T4 includes both protein-bound and unbound T4, while Free T4 measures only the unbound, active fraction. Free T4 is generally considered the more clinically useful measurement because it reflects the hormone actually available to the body's tissues.
Thyroid hormone levels fluctuate throughout the day. Testing before 10 am provides the most reliable and comparable results, especially if you are tracking levels over time.
Yes. Thyroid replacement hormones directly affect FT4 levels, and certain other medications, including corticosteroids and anti-seizure drugs, can also influence the result. Always inform your healthcare provider of all medications you take.