 In the early 1960s, desiccated thyroid hormones (thyroid extract) began to be replaced by levothyroxine (synthetic T4), or by combinations of T4 and T3.Replacement occurred faster in the United Kingdom than in North America, but by the 1980s more patients were being prescribed synthetic T4 (levothyroxine) or synthetic T4/T3 combinations than desiccated thyroid extract.Several reasons have been identified as to why prescriptions changed from desiccated thyroid treatment.