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Glossary term
Free T3 / Free T4
Also known as Free triiodothyronine - Free thyroxine - fT3 - fT4
The biologically active thyroid hormones. Free T3 in the bottom quartile often explains "fatigue with normal labs."
Free T4 is the storage form the thyroid releases; free T3 is the active form most tissues actually use, converted from T4 by deiodinase enzymes (primarily in the liver). Reference ranges: free T4 roughly 0.8-1.8 ng/dL, free T3 roughly 2.3-4.2 pg/mL. TSH and free T4 together can look "normal" while free T3 sits in the bottom quartile - especially in chronic stress, low-calorie dieting, or post-Hashimoto's states. Always pair free T3/T4 with TSH, reverse T3, and anti-TPO to get the full thyroid picture.
- Symbol
- Free T3 / Free T4
- Unit
- pg/mL / ng/dL
- Typical range
- Free T4 ~0.8-1.8 ng/dL; free T3 ~2.3-4.2 pg/mL.
Medically reviewed byDr. Lena Okafor, MD
Medical Director - updated April 2026How we review
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