Why are thyroid meds Meds causing debilitating headaches?

I'm actually asking for my Dad- He's been to sick to do research. He was diagnosed with Hypothyroidism 30 years ago( but is positive he had it for many years before that) so for the past 30 years he was on medication, I don't know which ones, but then he started getting such bad headaches that he can't move or think- I read it might be IIH, Idiopathic intracranial hypertension. I just found it in an artical about someone having this from thier meds, and my dad said he Had all the same symptoms. . On top of that he is hypoglycemic and if he can't eat his blood sugar drops and the headache doubles. He gets so sick he can't eat, can't move to eat. His Doc tried 3 other meds and all do the same thing. He's only OK if he stops taking them, but his Doc says that's not an option. Anyone have any ideas of what's going on and why for 30 years there was no problem?