I discovered I had an underactive thyroid about a year ago, but was stablized with 75 mcg, but when I quit smoking this January, it all went to hell. My numbers were off the charts (TSH over 15) and my Levo was increased to 100. Made it better for a bit, but then started feeling bad again. Went to an Endo and did a small increase of an additional 50 mcg of Synthroid a week-then to 112 and numbers were still off. Then tried Cytomel, 25 mcg along with 112 mcg of Synthroid and felt great for about two weeks, but now am back to feeling crappy again. Is it possible to build a tolerance to these drugs? It seems like everytime I increase or add something, I feel better for a couple of weeks and then slowly go back to where I was. It's been over 8 months of this and along with all of it, I've gained weight, but run 10 miles a week! Totally discouraged. Any help or suggestions?