no period CD 78

Becki
I'm on CD 78 and af still hasn't showed. I keep getting BFNs went to the doctors and got a negative blood test. found out it was my thyroids. I have an overactive thyroids but I'm on 75MG of thyroxine since start of last month. also got a faint positive followed by a negative. what could be wrong? doctors cant find anything wrong