outer‐ring (5′) deiodination to biologically active 3,3′,5‐triiodothyronine (T3) or by inner‐ring (5) deiodination to inactive 3,3′,5′‐triiodothronine (rT3). Sulfate conjugation is an important step in the irreversible inactivation of thyroidhormones. While sulfate conjugation of the phenolic hydroxyl group stimulates the 5‐deiodination of T4 and T3, it blocks the 5′‐deiodination of T4. We show that