Understanding the marker
Free T3 measures the unbound part of the thyroid hormone T3. It can add information when assessing thyroid overactivity, particularly when TSH is low.
This test addresses a specific thyroid question. It rarely adds value to a usual underactivity assessment when TSH and free T4 are already known. Illness and medicines can affect the result.
What does high Free T3 mean?
High free T3 may support thyroid overactivity when TSH and the other findings fit. Symptoms, medicines and possible test interference are considered before a diagnosis is made.
What does low Free T3 mean?
Low free T3 can occur during other illnesses as well as in thyroid-related patterns. TSH, free T4, symptoms and any treatment are needed to interpret it; this value alone does not establish an underactive thyroid.
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This is general information about the marker, not an assessment of your values and not medical advice. Last updated: October 2026.