What percentage of circulating T4 is converted to T3 in the body?

• A. 60%
• B. 80%
• C. 50%
• D. 90%

Answer: (B)

Most of the active thyroid hormone in tissues comes from converting the prohormone T4 into T3 outside the thyroid. This reaction is done by deiodinase enzymes (types I and II), which remove an outer-ring iodine from T4 to produce T3. Because of this peripheral conversion, about 80% of circulating T3 is derived from T4, while the thyroid itself adds only a smaller portion directly as T3. The remaining T4 is inactivated to reverse T3 by other deiodinases, allowing the body to adjust metabolic activity as needed. So the typical figure you’ll see is roughly 80% for T4-to-T3 conversion.