If a patient has a mutation causing constitutively active type 2 iodothyronine deiodinase (D2) in their astrocytes, which consequence follows in thyroid hormone regulation?

💡 Explanation

Constitutively active D2 in astrocytes increases local T3 production in the brain, which then suppresses TSH secretion via negative feedback at the hypothalamus and pituitary, because T3 inhibits TRH and TSH. Therefore, TSH decreases and T3 elevates, rather than TSH increasing, or T4 increasing with normal TSH.

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