What is the free fraction percentage of T3 in plasma?

• A. 0.4%
• B. 0.04%
• C. 4%
• D. 40%

Answer: (A)

The important idea is that thyroid hormones in blood are mostly bound to transport proteins, with only a tiny portion circulating freely to enter cells and exert effects. For T3, this free, active portion is about 0.4% of the total T3 in plasma. That means if total T3 were 100 ng/dL, roughly 0.4 ng/dL would be free, which corresponds to about 2–4 pg/mL, a typical free-T3 range.

This small free pool is why the correct value is 0.4%. Larger fractions would imply more unbound hormone than is physiologically observed, and percentages like 0.04% are more characteristic of T4’s free fraction rather than T3, while 4% or 40% would be far too high for a molecule mostly bound to proteins.