Free T3 Test
The free T3 (FT3) test measures triiodothyronine — the most biologically active thyroid hormone — in its unbound, available form.
It is less commonly ordered than TSH or free T4 but adds important information in specific situations.
Last medically reviewed: 1 November 2025
What it measures
The thyroid produces mostly T4, which tissues convert to T3 — the active hormone. Free T3 is the unbound fraction available to cells. Some patients have elevated T3 with normal T4 — "T3 toxicosis" — particularly in early hyperthyroidism or Graves’ disease.
FT3 can be affected by illness, fasting, and certain medications.
When it is ordered
FT3 is typically added when TSH is suppressed and free T4 is normal or borderline — to detect T3-predominant thyrotoxicosis. It may also be checked in monitoring certain treatments.
Routine screening usually uses TSH ± free T4; FT3 is a second-line test.
Interpretation
High FT3 with suppressed TSH suggests hyperthyroidism even if FT4 is normal — a pattern seen in early Graves’ and toxic nodular disease. Low FT3 with normal TSH/FT4 can occur in illness or calorie restriction and is often not clinically significant.
Results must be interpreted with TSH and FT4 together, not in isolation.
Frequently asked questions
Why would my doctor order FT3 if TSH and FT4 are done?
When TSH is suppressed but FT4 is normal, FT3 detects T3-predominant hyperthyroidism that FT4 alone would miss.
Can FT3 be high while FT4 is normal?
Yes — this is "T3 toxicosis," a recognised pattern in hyperthyroidism, particularly early Graves’ disease.
Is FT3 needed for hypothyroidism?
Rarely — hypothyroidism is usually assessed with TSH and FT4. FT3 may be checked in unusual cases or when converting between formulations.
References & further reading
This page is based on established medical knowledge and guidelines from authoritative sources. Always consult a qualified clinician for personal medical advice.
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