What Causes Hypnic Jerk Of The Body While Suffering From Hypothyroidism?

hello - i suffer from what i think is known as hypnic jerk, i.e. involuntary severe jerking of my body and extremeties when I am trying to relax. I also suffer simultaneously with electric shock/ buzzing sensations in my head. I suffer from GAD but am not receiving medication for this. I am hypothyroid and on daily thyroxine replacement therapy, for which i have regular blood tests. Is my jerking to do with my anxiety disorder or could it be that although my blood tests show that my thyroid chemistry is ok, in fact I might need more or less thyroxine?