7/29/2009 2:10:03 PM | Dr DavisRe your comment on what stops T4 to T3 conversion.Adequate selenium is required to form thyroid hormone and with the increasing amounts of heavy metals (which are selenium antagonists,)we all are absorbing, selenium levels are dropping.Selenium is a cofactor for 5????????-deiodinase, the enzyme required to convert T4 to T3.