Home › Lab markers › Free T4
METABOLIC & HORMONE MARKERS
Also printed as: FT4 · Free thyroxine · Free T4
Educational only — not medical advice.
Free T4 measures the thyroid hormone that is unbound and available to your tissues, as opposed to the much larger amount travelling attached to proteins. It is the more useful of the two measures because it reflects what your body can actually use. Your thyroid produces it in response to TSH.
Free T4 is what turns a TSH number into a picture. The relationship between the two — which is high, which is low, whether they agree — is what clinicians actually interpret.
TYPICAL ADULT RANGE
Many labs report roughly 0.8–1.8 ng/dL for adults. Ranges vary by lab and by assay method. Reported in ng/dL.
Reference ranges vary by lab, by method, and by age and sex. Always read your own result against the range printed on your report.
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A low free T4 usually appears alongside a high TSH, and that pairing is the classic pattern of lower thyroid output that your doctor will want to evaluate. Recent significant illness can lower it temporarily in a way that resolves on its own, and some medications affect it. Because thyroid decisions have real consequences, this is a result to review with your clinician rather than interpret from a report.
A high free T4 typically appears with a low TSH. Common explanations include thyroid hormone medication running a bit high, a temporary inflammatory phase of the thyroid, and pregnancy. Biotin supplements are a frequent source of falsely high readings and are worth mentioning to whoever ordered the test. Symptoms like palpitations, heat intolerance, or unintended weight change alongside this pattern are a reason to contact your doctor sooner rather than waiting.
Your doctor is the one who can read this number against your history, symptoms, and medications.
Educational only — not medical advice. Lab results only mean something alongside your history, symptoms, medications, and the rest of your panel. Bring questions to your doctor; they are the one who can interpret your results for you.
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