Abnormal tissue enzyme concentration or activity Reduced tissue type II iodothyronine deiodinase activity 2024-05-09T07:03:22Z Concentration or activity of type II iodothyronine deiodinase below the lower limit of normal. The activity can be measured in tissues such as cultured fibroblasts. Type II iodothyronine deiodinase is a selenoprotein that catalyzes the 5-prime deiodination of thyroxine (T4) to generate an active thyroid hormone, 3,3-prime,5-triiodothyronine (T3).