What Causes Constant Insomnia?

I am waiting on test results for my thyroid. trying to figure out reason for my insomnia for past 4 months. dr prescribed me amitriptyline 25 mg for my insomnia. I was told it was not an antidepressant id have to wean off of. but ive read differently online. the dr only gave me a 30 day supply. will insomnia go away after 30 days? if it is my thyroid what next? will they give me something else to take that will cure my problems?