T4 Production and Peripheral Conversion
The thyroid gland primarily produces T4 (about 80%) and a smaller amount of T3 (about 20%). T4 is a relatively inactive prohormone that serves as a reservoir. Deiodinase enzymes (D1 and D2) in the liver, kidneys, and other tissues convert T4 to active T3 by removing one iodine atom. This conversion process can be impaired by selenium deficiency, zinc deficiency, chronic stress (elevated cortisol), inflammation, and certain medications, resulting in adequate T4 but insufficient T3.
Prohormone, Not Active
T4 itself has limited biological activity. Its primary role is serving as a substrate for conversion to T3, the metabolically active thyroid hormone.
Conversion Can Be Impaired
Normal T4 with low T3 indicates a conversion problem, not a thyroid production problem. Root causes include nutrient deficiencies (selenium, zinc, iron) and chronic stress.
Optimal Free T4 Benchmarks
Common Questions
What if Free T4 is normal but I still have symptoms?
Check Free T3 and Reverse T3. You may have a T4-to-T3 conversion problem, where the thyroid produces adequate T4 but your body cannot efficiently convert it to the active T3 form.
Should I take T4-only medication like levothyroxine?
T4-only medication works well for most people, but some patients with conversion issues may benefit from combination T4/T3 therapy. Discuss with an endocrinologist and monitor both Free T4 and Free T3.