What this marker measures
Free T4 has largely replaced total T4 in modern practice because it is unaffected by changes in carrier protein levels. Total T4 is still used in some settings, particularly in paediatrics.
High or low: what it can mean
High Total T4
A high total T4 may reflect hyperthyroidism or elevated carrier protein levels (common in pregnancy or with oral contraceptives). Free T4 helps distinguish these causes.
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Low Total T4
Low total T4 may indicate hypothyroidism or reduced carrier protein levels. Again, free T4 provides a clearer picture of actual thyroid hormone availability.
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Frequently asked questions
What is Total T4?
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Total T4 measures both bound and free thyroxine in the blood. Most T4 is bound to carrier proteins and is inactive; only the free fraction is biologically active.
What does Total T4 measure?
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Free T4 has largely replaced total T4 in modern practice because it is unaffected by changes in carrier protein levels. Total T4 is still used in some settings, particularly in paediatrics.
What does high Total T4 mean?
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A high total T4 may reflect hyperthyroidism or elevated carrier protein levels (common in pregnancy or with oral contraceptives). Free T4 helps distinguish these causes.
What does low Total T4 mean?
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Low total T4 may indicate hypothyroidism or reduced carrier protein levels. Again, free T4 provides a clearer picture of actual thyroid hormone availability.
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.