Hello. As the package labeling indicates, if you are taking medication for thryoid disease, you should talk to your doctor before using alli. If you have already spoken to your doctor, and they are aware you are using alli, you can use it. As far as dosing, it is always best to take your Synthroid dose first thing in the morning, and wait about 1/2-1 hour before eating or taking other medications. If you normally do that, you can take alli with your breakfast...which would be at least 1/2 hour after your Synthroid.
Hope this helps!
Maria Marzella Sulli, Pharm.D.