Ajax Harwood Clinic
Free T4 (thyroxine)
This test is useful only in specific situations — it's not right for everyone who asks about it.
What this test is
Free T4 is a thyroid hormone that flows in your blood. Doctors use it together with TSH, not by itself. It is checked when a TSH result is not normal, to help find out what is going on. It is also used for a rare thyroid problem linked to the pituitary gland, a small gland near your brain. If you already take thyroid pills and your TSH check is enough, adding free T4 every time does not usually change your dose. Free T4 is a useful test. It is just not the first test, and it is not needed at every visit.
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.