How Good Is Levothyroxine After Taking It For Over 15

How good is levothyroxine after taking it for over 15 years? I also have taken Glimiperide about the same lingth of time. I have been experiencing many of the side effects for years and my Dr. doesn't seem to take me seriously... even though some of the side effects are quite visible. Some are muscular, and some are also mental. Any info on this would be great. I'm seriously stopping both of these and going holistic. At 71, I'm not about to give in to these problems!
Thanks.