I've been on natural thyroid replacement for about 6 years during which time I suspected I also had B12 deficiency but couldn't get anywhere with my doctor. I started self-injecting in October last year and suddenly I find that I seem to be taking too much thyroid medication as my heart beat is faster plus other symptoms. I have reduced them and my heart beat has slowed down a bit. It suddenly occurred to me that initially, perhaps my thyroid was compromised from having a B12 deficiency. Could B12 therapy kick my thyroid back into life? I am seeing my doctor next week.