I am also hypo and have experienced hives. I went to my PCP for several weeks and all he would prescribe was allergy medicine (allegra, and zyrtec) The allergy stuff didn't do anything for me. I finally went to our family dermatologist for relief. He perscribed prednesone and the hives cleared up immidiately, in fact I had some relief by that evening. Of course he wanted to know what other meds I was taking but it didn't seem to be a big deal that I was on Synthroid. I don't know if your situation is similar to mine, but you may want to try a dermatologist, it sure worked for me.