Is it safe to take plexus slim if I am taking Synthroid (thyroxine) 150mcg?

Yes. Plexus slim is a mishmash of soluble fiber (to make you feel full), stuff from beets, grapes, and common chemicals that are probably harmless. Basically nothing interacts with thyroid replacement like synthroid (thyroxine) in any case. It's your decision; the most successful weight-losers are the ones who find an aerobic sports activity that they enjoy. Good luck.