Awhile back, when Alli was getting ready to be released, I had mentioned that I had asked my ob/gyn if it would be safe to take with my thyroid condition. She had told me that it was also known as Orlistat, and was a fat blocker, that I needed to be able to deal with the possibility of oily stools that could come out on it's own. She also said that it would be safe. I never did try it, simply because I wasn't sure about the whole 'oily stool' thing.
Maybe it's a good thing, because the FDA just approved a warning label that is to be put on Alli and Xenical stating that it can decrease levothyroxine absorption and cause increased hypothyroidism. This is not to say that you can't safely take Alli if you are hypothyroid to help lose weight, but to say that those on thyroid medication should be careful. It is advised that Alli be taken four hours apart from thyroid medication, and that thyroid levels should be monitored to make sure that the medications don't need adjusting.