The majority of thyroxine (T4) is converted into the more biologically active hormone:

• A. Thyroglobulin
• B. Thyroid-stimulating hormone
• C. Triiodothyronine
• D. Thyrotropin-releasing hormone

Answer: (C)

The key idea is that most of the biologically active thyroid hormone comes from peripheral conversion of thyroxine (T4) to triiodothyronine (T3). In tissues like the liver and kidney, deiodinase enzymes remove an iodine atom from T4 to form T3, which binds thyroid hormone receptors with much higher affinity and drives the majority of T3-mediated metabolic effects. This makes T3 the active hormone, while T4 serves mainly as a prohormone that is converted where needed. Some T4 can also become reverse T3 (inactive) in certain conditions, but the hormone responsible for the main activity is T3. The other options refer to a storage precursor (thyroglobulin) and pituitary/hypothalamic signals (TSH and TRH), not products of T4 conversion.