The D3 deiodinase enzyme inactivates T4 and T3 to prevent excessive thyroid activity. When T4 and T3 are sufficiently circulating in the bloodstream, the hypothalamus and pituitary reduce TRH and ...

or even consumptive hypothyroidism as a result of induction of the thyroid-hormone-inactivating type 3 deiodinase. Although this adverse effect can occur in any levothyroxine-treated patient ...

(7) The difference in deiodinase in the pituitary makes it super sensitive to T4 and T3. So it's possible, and common, that ...