help my sisiter

I am worried about my sister. She is 39 years old, very skinny, and seems to have slurred speech at night. She has an underactive thryroid, and the explaination we have been given for the slurred speech is that she has forgotton to take her thyroxine... I tend not to believe this, as I have the same condition, yet if I miss my medication, I do not slur. And also, because she is so skinny, if she missed her pills she would put on weight.
I would go as far as to say, she is borderline anorexic. could the slurred speech be caused by low blood sugar level?! But first I would like a yes/ no answer to that first question, "could the slurring be caused by not taking thyroid medication?"
Thank you!