I posted the following reply to your other post, before I noticed your new thread here. So here is the same info.
Why are you sure the hives started from the thyroid med? Is it possible that the cause for being on Levo and Synthroid was that you were diagnosed with Hashimoto's Thyroiditis, with which the autoimmune system erroneously identifies the thyroid gland as foreign to the body and produces antibodies to attack and eventually destroy the gland? If so, Hashi's antibodies have been identified as causing something referred to as Hashi's rash.
The fact that you still have hives 2 years after stopping the thyroid med seems to indicate it was not related to that. What other symptoms do you have, if any? Do you have thyroid related test results from prior tests that you can post, along with reference ranges?