Thyroid health
Free thyroxine (free T4)
FT4
Included in Baseline and Complete.
Compare membershipsFree T4 tracks the main hormone your thyroid releases.
T4 is the main hormone released by your thyroid, and your body converts it into the more active T3. Free T4 measures the portion that isn't bound to proteins and is available to your tissues.
TSH and free T4 work as a pair.
When free T4 falls, the pituitary releases more TSH to prompt the thyroid. When free T4 rises, TSH drops. Reading the two together helps show whether a change starts in the thyroid itself or, less often, in the pituitary.
Free T4 is generally preferred over total T4 because it reflects the hormone your tissues can use. Pregnancy and estrogen-containing medicines such as birth control pills change the proteins that carry T4, which affects total T4 most. Severe illness can affect T4 results too.
Free T4 shows the supply. TSH shows how hard the pituitary is asking.
Questions
What do low or high free T4 results mean?
A low free T4 with a high TSH may suggest an underactive thyroid. A high free T4 with a low TSH may suggest an overactive one. Other patterns are less common and worth reviewing with a clinician, who will consider your symptoms, medicines, and history.
Can supplements affect my result?
Yes. Biotin supplements can interfere with some thyroid lab tests and make results look abnormal when they aren't. The American Thyroid Association suggests not taking biotin for two days before thyroid blood work, so check with your clinician.
Is free T4 useful if my TSH is normal?
In most people, a TSH within the reference range points to normal thyroid function. Choosing Wisely Canada advises against using free T4 to screen for an underactive thyroid. Free T4 adds the most when TSH is abnormal or a pituitary problem is suspected.