What is Free T4?
Free T4 is the active, unbound form of thyroxine — the primary hormone produced by the thyroid gland. It is the fraction available to enter cells and exert its effects.
Free T4 is measured alongside TSH and sometimes Free T3 to build a complete picture of thyroid function. TSH is the more sensitive screening test; Free T4 is used to confirm and characterise abnormalities found on TSH.
Why might Free T4 be low?
Low free T4 indicates reduced thyroid hormone production (hypothyroidism). This is usually accompanied by an elevated TSH as the pituitary tries to stimulate the thyroid.
This information is educational. A single out-of-range result is rarely the whole picture — a healthcare professional can interpret your numbers alongside your overall health and history.
Context questions that matter
- →Was this result slightly or clearly outside the reference range?
- →Were related markers also high or low?
- →Is this new compared with previous tests?
- →Could recent illness, exercise, fasting, alcohol, supplements, or medication affect it?
- →Do you have symptoms that should be discussed with a clinician?
Markers often reviewed together with Free T4
Clinicians rarely look at one number in isolation. These markers usually accompany Free T4 on a thyroid panel and help put the result in context.
Questions to ask your doctor
- →How does this result fit with TSH, Free T4, Free T3 and thyroid antibodies?
- →Could timing, medication, supplements such as biotin, or recent illness affect this result?
- →Should this be repeated, or interpreted with my symptoms and previous thyroid results?
Don't guess from one flagged result.
Start with the most relevant panel and see how your markers fit together in plain English.
Frequently asked questions
What does low Free T4 mean?
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Low free T4 indicates reduced thyroid hormone production (hypothyroidism). This is usually accompanied by an elevated TSH as the pituitary tries to stimulate the thyroid.
What is Free T4?
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Free T4 is the active, unbound form of thyroxine — the primary hormone produced by the thyroid gland. It is the fraction available to enter cells and exert its effects.
Should TSH be read with Free T4 and Free T3?
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Thyroid markers are usually interpreted together. TSH, Free T4, Free T3 and sometimes thyroid antibodies (TPO, Tg) provide more context than any single result, particularly when TSH is borderline.
Can supplements or timing affect thyroid results?
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Time of draw, thyroid medication timing, acute illness, pregnancy and supplements such as biotin can affect some thyroid assays. If a repeat test is suggested, ask your doctor how to prepare so the two values are comparable.