Do you mean 100mcg T4? And does your doc know about the Clen?
100mcg is a standard, mid range dose for replacement therapy in many cases, but beta agonists do not generally stack cleanly with Thyroid hormones.
In fact, with higher doses of T4, a cardio selective beta-blocker (the opposite effect of clen) is often used to control excessive cardiac acceleration.
You might be alright with just 100 though, if you don't get greedy with the clen.