T4 to T3 conversion

Enzymatic process by which thyroxine (T4), the main thyroid-produced hormone, is converted to triiodothyronine (T3), the biologically active form. Performed by deiodinases (mainly DIO1 and DIO2), mainly in the liver, kidneys, muscles and brain. Conversion depends on selenium (essential deiodinase cofactor) and can be altered by stress, chronic diseases, certain medications and very restrictive diets. A conversion deficit can cause hypothyroidism symptoms despite normal TSH.