Are you breaking out in hives & don't know why?

Are you breaking out in hives & don't know why?  I had severe hives.  After months I was diagnosed with a thyroid autoimmune disease & put on many medications.  In the end, Zyrtec (which you can now buy in generic form) was the only thing that got them under control.  I quit taking it after a few years without a problem, however lately the hives are occurring once again.  Today it was after I ate some chicken, so now I'm reading about the hormones & antibiotics found in chicken that could have caused my reaction.  No, it was not Tyson chicken.  It was Foster Farms, which prides itself on what they don't feed their chickens.  Could the animal proteins be creating the hives reaction?  Guess it's back to Zyrtec and a vegetarian diet.  My hives were so severe previously that I ended up living off of green apples and water.  Appears I'll need to do this again.  It's been 5 months since I had a flu shot for those of you who think this may be your trigger.  It took a blood screen to find the thyroid autoimmune disease.