What this marker measures
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.
High or low: what it can mean
High Free T4
Elevated free T4 suggests the thyroid is producing more hormone than the body needs (hyperthyroidism). This is typically accompanied by a low TSH.
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Low Free T4
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.
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Frequently asked questions
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.
What does Free T4 measure?
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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.
What does high Free T4 mean?
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Elevated free T4 suggests the thyroid is producing more hormone than the body needs (hyperthyroidism). This is typically accompanied by a low TSH.
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.
Related biomarkers
Editorial note
LabTestly explains lab markers using plain-English educational content. Reference ranges may vary by laboratory and should be interpreted with a clinician.
Educational content · Editorially reviewed for clarity · Last updated
Common sources used for lab-test education
Not medical advice
This page is for general understanding only and does not replace medical advice from your clinician. Reference ranges can vary by laboratory, age, sex, pregnancy status, medications, and clinical context.