What this marker measures
Free T3 is not routinely measured in initial thyroid screening because TSH and Free T4 are usually sufficient. It becomes particularly useful when diagnosing T3 toxicosis or monitoring patients on T3-containing thyroid medications.
High or low: what it can mean
High Free T3
Elevated free T3 is seen in hyperthyroidism and certain conditions where T4-to-T3 conversion is increased. Symptoms can include rapid heart rate, weight loss, anxiety, and sweating.
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Low Free T3
Low free T3 can occur in hypothyroidism or in non-thyroidal illness syndrome (euthyroid sick syndrome), where T3 drops during serious illness even when the thyroid is functioning normally.
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Frequently asked questions
What is Free T3?
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Free T3 is the most biologically active thyroid hormone. Most T3 is produced by converting T4 in peripheral tissues, not directly from the thyroid gland.
What does Free T3 measure?
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Free T3 is not routinely measured in initial thyroid screening because TSH and Free T4 are usually sufficient. It becomes particularly useful when diagnosing T3 toxicosis or monitoring patients on T3-containing thyroid medications.
What does high Free T3 mean?
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Elevated free T3 is seen in hyperthyroidism and certain conditions where T4-to-T3 conversion is increased. Symptoms can include rapid heart rate, weight loss, anxiety, and sweating.
What does low Free T3 mean?
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Low free T3 can occur in hypothyroidism or in non-thyroidal illness syndrome (euthyroid sick syndrome), where T3 drops during serious illness even when the thyroid is functioning normally.
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.