Total T4 Test
The total T4 test measures all thyroxine in the blood — bound to carrier proteins plus the small free fraction.
Free T4 is generally preferred for assessing thyroid status, but total T4 has specific uses.
Last medically reviewed: 1 November 2025
What it measures
Most T4 in blood is bound to proteins like TBG (thyroxine-binding globulin); only ~0.03% is free and active. Total T4 includes both fractions.
Because the bound fraction varies with protein levels, total T4 can be misleading when binding proteins are abnormal.
When it is used
Total T4 is sometimes used in specific contexts — for example, older assays or when free T4 is unavailable. It is occasionally used to calculate binding capacity or in certain monitoring protocols.
In routine practice, free T4 has largely replaced it for diagnosing thyroid dysfunction.
Limitations vs free T4
Total T4 can be falsely high in pregnancy, estrogen therapy, liver disease, or with certain drugs that increase binding proteins — even when thyroid function is normal. Conversely, it can be falsely low in protein-deficiency states.
Free T4 measures only the active fraction and is more reliable for assessing thyroid status.
Frequently asked questions
Why did my doctor order total T4 instead of free T4?
Usually free T4 is preferred. Total T4 may be used in specific situations or older protocols — ask your clinician if you are unsure.
Can total T4 be wrong?
Yes — protein-binding variations can make it misleading. If results seem discordant with symptoms, free T4 provides a clearer picture.
Is total T4 needed during pregnancy?
Pregnancy alters binding proteins significantly — free T4 (with trimester-specific ranges) is preferred for accurate assessment.
References & further reading
This page is based on established medical knowledge and guidelines from authoritative sources. Always consult a qualified clinician for personal medical advice.
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