Both are different BRANDS of T3
Cynomel is a mexican brand name: USV Grossman (see pic)
Cytomel is a US/European brand name; made by Uni-Pharma, King Pharmaceuticals, JPI Jones etc
now, IMO there is a difference in potency..
for me the cynomel did crap, but the Uni-pharmas i had were kick ass.. so that is the main difference i have personally seen...
Super