During an endocrinology lecture, a professor explains why thyroxine remains in the bloodstream longer than triiodothyronine. Which feature best accounts for this difference?

• A. Thyroxine is secreted only during stress, whereas triiodothyronine is released continuously.
• B. Thyroxine binds less strongly to plasma proteins, causing slower tissue uptake.
• C. Thyroxine has a longer half-life and mainly serves as a circulating precursor that can be converted into the more active triiodothyronine.
• D. Thyroxine contains fewer iodine atoms, making it less susceptible to enzymatic breakdown.

Answer: (C)