You pretty well answered your own question. I have tried to stop my beta blocker med for irregular rhtymn from time to time but after a few days the symptoms reappear. It takes about 3 or so days but it does reoccur. My wife takes Synthroid and if she misses it for a few days or even two days, she gets sluggish and tired. Her thyroid has had most of it removed, so she's pretty dependent on the meds. Is your thyroid related to your heart condition?