Free T3 (triiodothyronine) is the active thyroid hormone — the form that enters cells, binds to nuclear receptors, and directly regulates gene expression and metabolic processes. While the thyroid produces some T3 directly, approximately 80% of circulating T3 is produced by the conversion of T4 to T3 in peripheral tissues.

Some individuals have impaired T4-to-T3 conversion due to genetic variants, nutrient deficiencies (selenium, zinc, iron), chronic illness, or elevated reverse T3 — and may experience hypothyroid symptoms despite having normal TSH and free T4. Measuring free T3 directly assesses the active hormone level and is particularly important for people on T4-only thyroid medication (levothyroxine) who continue to experience symptoms despite normalised TSH.