Low thyroid after anorexia?

I've recently been through anorexia, had the counselling. my family recognised symptoms almost immediately and now i'm thankful but whilst at the doctors getting a blood test they found something else, that i have low thyroid. my doctor was surprised as my bmi is 19.8, the disease could have been set off by anorexia but it is also hereditary. thing is, I've missed about 4 menstrual cycles now and the doctor said that it should be here in a month. if its not, i don't know what ill do, im so scared. has anyone had this experience? i need someone to tell me everything will be okay