I was on Nature Throid for 4 years and just made the switch to Synthroid because I was having a lot of issues with heart palpitations and just never feeling good. I was on 81.25 mg Nature Throid and my doctor put me on 100 mg of Synthroid. I've been feeling good, BUT my chest feels heavy on the left side. I started to feel it a bit on the first day and now I'm on day 3 and it's still feeling heavy. I'm breathing ok, but wondering if I'm being over medicated. I'm thinking he should've given me the 75 or 88.