I take Synthroid daily after being treated for autoimmune thyroid disease. I was on the Synthroid before I developed UC so can't give you personal experience, but from my research I'm not aware of any interactions.
Like you, I couldn't take Asacol and Colazal didn't provide adequate symptom relief. I've taken all of the 5-ASA's, Azathioprine, Prednisone, rectal steroids and Remicade while on Synthroid and haven't noticed any problems.
A word of warning: be sure you do take the Synthroid, because for those of us with minimal or no thyroid left, not taking our medication can be very serious and long-term it can be fatal.
Moderate to severe left-sided UC diagnosed 2001.
Flared for 5 years, finally in remission with Remicade since March 2006.
Avascular necrosis in both shoulders is my "forever" gift from steroids.
Colazal, Remicade, Nature's Way Primadophilus Reuteri.
"My life is an ongoing medical adventure"