A patient with a thyroid tumor exhibits chronically elevated TSH levels despite high doses of exogenous thyroxine. What explains why thyroxine fails to suppress TSH secretion?

💡 Explanation

Thyroxine (T4) is converted to T3, which provides negative feedback on TSH release. If conversion is impaired, feedback is lost, because the pituitary is less sensitive to T4 rather than only T3. Therefore, the tumor drives TSH release despite high T4.

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