If a patient with iodine deficiency receives a fixed daily dose of levothyroxine (synthetic T4), why does serum T3 concentration remain lower than expected despite normalized TSH?

💡 Explanation

Low iodine impairs deiodinase enzymes, limiting peripheral T4 to T3 conversion, because these enzymes require selenium as a cofactor, whose activity is impaired without sufficient thyroid hormone. Therefore, T3 remains low, rather than T3 clearance increasing or T4 uptake being inhibited directly.

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