It is impossible for your healthcare provider to know whether you will develop insomnia while taking levothyroxine. Also, in some cases, if a person does develop insomnia, it is difficult to determine if these problems are because of other, unrelated reasons. The best way to determine whether levothyroxine is causing your insomnia is with a simple blood test.
If you develop insomnia while taking levothyroxine, make sure to talk with your healthcare provider. He or she may be able to recommend other treatment options, such as adjusting your levothyroxine dosage or suggesting a sleep medication.