What this marker measures
rT3 is not part of standard thyroid screening and is mainly ordered in research or specialised settings. There is ongoing debate about its clinical utility; most clinicians do not order it routinely.
High or low: what it can mean
High Reverse T3
High rT3 is most commonly seen in non-thyroidal illness ("euthyroid sick syndrome"), prolonged fasting, severe stress, or critical illness, where the body diverts T4 conversion toward the inactive form. It is not by itself diagnostic of thyroid disease.
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Low Reverse T3
Low rT3 is uncommon and rarely clinically significant on its own. Interpretation should be in the context of TSH, free T4, and free T3.
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Frequently asked questions
What is Reverse T3?
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Reverse T3 is an inactive metabolite of thyroxine (T4). The body converts some T4 to active T3 and some to inactive rT3 — the ratio shifts under physiological stress.
What does Reverse T3 measure?
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rT3 is not part of standard thyroid screening and is mainly ordered in research or specialised settings. There is ongoing debate about its clinical utility; most clinicians do not order it routinely.
What does high Reverse T3 mean?
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High rT3 is most commonly seen in non-thyroidal illness ("euthyroid sick syndrome"), prolonged fasting, severe stress, or critical illness, where the body diverts T4 conversion toward the inactive form. It is not by itself diagnostic of thyroid disease.
What does low Reverse T3 mean?
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Low rT3 is uncommon and rarely clinically significant on its own. Interpretation should be in the context of TSH, free T4, and free T3.
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.