TSH vs free T3 — which gives a better picture of thyroid function?
TSH is the most sensitive screening marker. Free T3 (and free T4) tell you what is actually circulating and reaching tissues. In symptomatic patients with borderline TSH — or on thyroid replacement — reading both is the standard of care.
Full answer
TSH and free T3 answer different questions. TSH is the pituitary's signal to the thyroid and the most sensitive single screening marker for thyroid dysfunction — it changes earlier than circulating hormone levels and is the right first test. But TSH does not tell you what is actually circulating; for that you need free T4 (the predominant circulating form) and free T3 (the metabolically active form, mostly converted from T4 in peripheral tissues). In symptomatic patients with a borderline TSH, in patients on thyroid hormone replacement, and in patients with suspected conversion or peripheral resistance issues, free T3 and free T4 are essential — TSH alone misses these. Provider review reads TSH, free T4, free T3, reverse T3, and TPO antibodies together for the complete picture.
Educational. Not a substitute for individualized provider review.
- Free T3 — Primary source · oMeds
- Thyroid intelligence — Reference · oMeds Learn
- Reviewed by Dr. Laura Purdy, MD, MBA · Clinical Director, oMeds · Last reviewed 2026-05-29