I've been on Levoxyl for 9 months now, no weight gain. I don't think weight gain is a side effect of the medication, but of the condition. You may not be on the right dose, or like the other post said, just getting your appetite back. You need to talk with your Dr, and find out why the weight gain. Being hypo will slow down your metabolism, but not all hypos gain weight. I was very hypo for 8 weeks waiting to be scanned, and I actually lost 4lbs. Crazy!! Good luck on getting well.