Hypothyroidism is characterized by too little thyroid activity, with symptoms of weight gain, fatigue, and sensitivity to cold. Hyperthyroidism is characterized by too much thyroid activity, with symptoms of weight loss, accelerated heart weight, and sensitivity to heat. Hypothyroidism is the more common one.
Dr. Graham R. Williams and Dr. J. H. Duncan Bassett of Imperial College London, UK, provided a review of thyroid hormones. The thyroid produces primarily T4 (tetraiodothyronine or thyroxine). The name comes from the four iodine atoms in the molecule. T4 is actually a pro-hormone, which is not physiologically active.
Deiodinase enzymes convert T4 to the active hormone T3, also called 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.
https://www.empowher.com/hypothyroidism/content/t3-t4-and-tsh-all-important-thyroid-function