Deiodinase enzymes convert T4 to the active hormone T3 (triiodothyronine) by removing one iodine atom. A similar enzyme inactivates T3 by removing another iodine atom to form T2. The thyroid also produces small amounts of T3.
Hypothyroidism is treated by replacement T4 given orally. However, the deiodinase enzymes may be just as important, according to Williams and Bassett. “Ultimately, an important challenge will be to exploit DIO2 [a deiodinase enzyme] as a drug target to manipulate tissue thyroid status.”
They explained that production of T4 is controlled by thyrotropin-releasing hormone (TRH), produced by the hypothalamus, and by thyroid stimulating hormone (TSH), produced by the pituitary gland. TSH is the most commonly measured indication of thyroid function.
When T3 and T4 levels in the blood drop, the pituitary produces more TSH, a signal to the thyroid to produce more T4. When T3 and T4 levels rise, the pituitary produces less TSH.
https://www.empowher.com/hypothyroidism/content/t3-t4-and-tsh-all-important-thyroid-function