DDH - the experience I draw from with a thyroid issue was my darling fur-baby who was hyperhtyroid in his last few years. I hope you don't mind. Anyway, his poor heart really took the brunt of it. With his thyroid cranking, his heart worked overtime and became thickened (cardio myopathy) and, well, I don't remember all the things wrong with it, but the heart specialist vet told us after the ultrasound that those three problems can't all exist in one heart. He truly was on borrowed time that last couple of years.... From what they told me, I gathered that the extra work his heart was doing, just becuase his thryoid was making it run faster, overdeveloped the muscle. The muscle tissue gets so thick that the heart eventually doesn't do it's job properly as it sort of gets in the way of itself and also loses it's elasticity.

Based on the little I know about it, I'm thinking your endo doc is concerned that the more your heart works at that higher HR, the more potential there is for thickening. While the treatments should get your thyroid back to normal, it's possible that the damage it's doing to your heart in the meantime might not be reversible. Your body is precious - every part of it. Sometimes, as hard as it seems, you need to do just the opposite of what you think you need to do, to give it a chance to heal.

Sending you butterflies of patience, calm, healing and hope. You'll get through this.

Hugs,
~T~

ps - if you are interested in an alternative, natural method of testing and balancing your systems, that can work in conjunction with your medical treatment, let me know, and I'll tell you my own story and find someone in your area you can see.