What Causes Episodes Of Hot Flashes At The Age Of 84 Post Hysterectomy?

What causes periods of hot flashes in an 84 year old woman who is healthy and has had a histerctomy years ago. my dr. Has diagnosed an overactive thyroid gland. I was treated for a goiter with radio--active iodine in 1951 with very good results. But recently the thyroid gland has become overactive and my dr. Is trying to adjust it with levothyroxin. But I am stili having periods of shakiness and perspiration(hot flashes) which last about 15 to 20 minutes. Any suggestions? I would appreciate your advice. Thank you in advance. Nancy