Why does peripheral tissue hypothyroidism develop despite normal TSH levels after prolonged amiodarone use?

💡 Explanation

Amiodarone inhibits the deiodinase enzyme, preventing T4 to T3 conversion in peripheral tissues, leading to localized hypothyroidism even with normal TSH because the hypothalamus responds primarily to circulating T4 levels rather than peripheral T3 levels. Therefore, T3 levels are reduced peripherally, rather than via altered TSH receptor sensitivity or increased hormone excretion.

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