I have also developed hives, each time after eating Activia yogurt for the past several weeks. Mine are on my legs and arms, and do not go away quickly, but do not itch like bug bites or allergies, but some spread like a spider bite. I am also on synthroid which I initiatially thought might be the cause, but I've been taking it for years and never had any problems. I've never had hives before, and the only thing new I've eaten is Activia, which I won't be eating any more! Thank you for your post, so I know I'm not crazy!! I would like to know what causes this reaction.