I had a complete thyroidectomy because of thyroid cancer. I take synthetic t4 (Tirosint). T4 is very slow acting. If my synthetic t4 dose is changed, it takes ~ 2 weeks for me to notice and 6 -8 weeks for my thyroid hormone levels to stabilize. It is not a PED. If someone is using synthetic t4 to help mitigate the effects of PED use, they would have to have a good medical support system. Timing and dosing synthetic t4 with PED's would be a non-trivial exercise.
Synthetic T3 in small doses might be a ped. It is very fast acting and if dosed appropriately might produce the desired effects - short term weight loss and a psychological boost. If someone is taking too large a dose of synthetic t4, AFIB is one of the potential side effects. I don't know if too large a dose of synthetic t3 would induce AFIB. The idiotic Dutch cyclists who fooled around with EPO should give people reason to pause.