Hello! Levothyroxine is a natural precursor of thyroxin. Normally it is produced by the thyroid gland and it turns in thyroxyn which is the active hormone of thyroid gland, and all the welllnown actions of thyroid gland are attributed and performed by this hormone. When the thyroid gland has low function and doesn t produce enough hormones, those have to be replaced. Thyroxin is a substance that dissolves fast, so it can be used as an external substitute. Meanwhile, levothyroxine has a long half life, so its presence in bloodstream is long and its effects can be dose controlled. If hypothyroidism is caused by lack of iodine, this last one should be taken. But if your doctor check your blood levels correctly and the results show hypothyroidism, you must use levothyroxine. In general when someone need levothyroxine, it has to be taken lifelong and should be consulted by the doctor if a change in dose is required. I hope i have been of help