Biomarker · Thyroid
Free T3 (free triiodothyronine)
T3 is the most active thyroid hormone. When a T3 or free T3 test is ordered, how it adds to TSH and T4, and why it is not a routine first-line test.
Written by LON6EVITY Editorial Team · Last updated
What T3 is
Triiodothyronine (T3) is the more active form of thyroid hormone. Much of it is converted from T4 in body tissues. Like T4, most T3 circulates bound to proteins and a small free fraction is active.
When it is ordered
According to MedlinePlus, T3 tests are mainly used to help diagnose hyperthyroidism (an overactive thyroid), often after an abnormal TSH result. T3 is less useful for diagnosing an underactive thyroid, because T3 levels can stay in range until hypothyroidism is advanced.
Interpreting results
T3 results are read alongside TSH and free T4. Illness, fasting and certain medicines can lower T3 without thyroid disease, so an isolated low T3 on its own needs clinical context.
Reference ranges
Reference ranges differ between laboratories, units and methods, and depend on age, sex and your overall clinical picture. A single result is not a diagnosis — interpret it with a qualified clinician.
Factors that can affect the result
- Hyperthyroidism
- Acute or chronic illness
- Medicines
- Pregnancy
Questions people commonly ask
- Should T3 be part of a routine check-up?
- Usually not. TSH is the usual starting test; T3 is added when a clinician suspects hyperthyroidism or needs more detail.
Related biomarkers
Related tests
Understand your biomarkers in context
What does this mean for your own health data? Log your results, follow trends over time and bring an organised summary to your next doctor's appointment.
Sources
- Triiodothyronine (T3) Tests. MedlinePlus, U.S. National Library of Medicine.
- Thyroid Tests. National Institute of Diabetes and Digestive and Kidney Diseases (NIH).