Which statement about T3 production is correct?

• A. Direct secretion by thyroid gland
• B. Most T3 is produced by monodeiodination in non-thyroid tissues
• C. T3 is produced in the pituitary
• D. T3 is produced only in the liver

Answer: (B)

Most circulating T3 comes from peripheral conversion of T4 to T3 by outer-ring deiodinase enzymes in non-thyroid tissues such as liver and kidney. This monodeiodination turns the circulating, longer-lasting T4 into the more active T3, which explains why T3 is predominantly produced outside the thyroid. The thyroid does secrete some T3 directly, but that amount is small compared to the T3 produced in peripheral tissues. The pituitary isn’t a primary source of circulating T3, though it can generate local T3 to help regulate its own function; and T3 production is not limited to the liver—several tissues contribute to this conversion. So the statement that most T3 is produced by monodeiodination in non-thyroid tissues is the best description.