Ajax Harwood Clinic
Free T3 (triiodothyronine)
This test is useful only in specific situations — it's not right for everyone who asks about it.
What this test is
Free T3 is a thyroid hormone test. T3 is the active form your body uses. Most of the time, TSH alone tells your doctor what is needed. Free T3 does not add much extra information. It has two narrow uses. One is checking a rare type of overactive thyroid where only T3 is high. The other is helping sort out a rare pituitary-related thyroid problem. Feeling tired or gaining weight with a normal TSH is usually not caused by low free T3. A low free T3 alone, with a normal TSH, does not have a proven treatment.
Want to see the full clinical reasoning? The reference we use for this test is open to anyone — it is detailed and written for clinicians, and you are welcome to read it.
Open the clinician reference →Looking for other tests, or more on a condition an ND raised? Explore ask.ajaxharwoodclinic.com →
General health information for Ajax Harwood Clinic patients and the public — not medical advice, and not a substitute for seeing your own doctor. Last reviewed 2026-09-24.