A patient with Hashimoto's thyroiditis takes levothyroxine (synthetic T4). Which feedback mechanism impairment most likely reduces the drug's effectiveness in restoring normal T3 levels in peripheral tissues?

💡 Explanation

Impaired peripheral T4 to T3 conversion limits the amount of active T3 available, because levothyroxine primarily replaces T4, which must be converted. Therefore, even with adequate T4 supplementation, T3 levels remain low. Rather than other thyroid-specific effects, the issue lies in peripheral conversion by deiodinases.

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