What this marker measures
Total T3 is not a routine screening test. Free T3 is preferred when T3 measurement is specifically indicated, as it is unaffected by binding protein levels.
High or low: what it can mean
High Total T3
Elevated total T3 is the defining feature of T3 toxicosis, a form of hyperthyroidism where only T3 is raised while T4 remains normal. It is also elevated in more typical hyperthyroidism.
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Low Total T3
Low total T3 can be seen in hypothyroidism or non-thyroidal illness syndrome. T3 often drops during serious illness as a protective adaptive response.
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Frequently asked questions
What is Total T3?
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Total T3 measures all triiodothyronine in blood, including both protein-bound and free fractions. It is occasionally ordered alongside TSH and T4 when T3 hyperthyroidism is suspected.
What does Total T3 measure?
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Total T3 is not a routine screening test. Free T3 is preferred when T3 measurement is specifically indicated, as it is unaffected by binding protein levels.
What does high Total T3 mean?
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Elevated total T3 is the defining feature of T3 toxicosis, a form of hyperthyroidism where only T3 is raised while T4 remains normal. It is also elevated in more typical hyperthyroidism.
What does low Total T3 mean?
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Low total T3 can be seen in hypothyroidism or non-thyroidal illness syndrome. T3 often drops during serious illness as a protective adaptive response.
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
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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.