Just started synthroid (thyroxine) 100mcg (today is the 3rd day) for hypothyroidism, tsh was 10.85. last night and tonight i'm very agitatated, is this expected?

It can be......: You are taking this in the morning, right? If not, it is a morning medicine. Still, as you're adjusting to being "at the right speed" so to speak, it can make you feel agitated. Talk to your dr. About a short term medicine for sleep if you're having trouble. You may also want to ask your dr. If you can start with 50 mcg for a couple of weeks instead.