I didn't notice sulfasalazine causing any problems with my Synthroid, but my endocrinologist has told me before that when I change meds, I need to see her because they can interfere with the Synthroid and throw off my levels. I thought she meant my biologicals, but I guess a lot of other meds could probably interfere as well. But, I didn't stay on sulfasalazine long because it hurt my stomach, so maybe that's why I didn't have any problems.