I get aquagenic urticaria (water hives) with too much thyroid hormone in the blood. Even just a mild amount of excess thyroid hormone will cause this.
"Elevated thyroid antibodies are often associated with chronic urticaria, also called hives. Studies report that as many as 57.4% of patients with hives have the presence of anti-thyroid antibodies. An August 2010 paper suggests that treatment with T4 improves the itching associated with urticaria, but did not advise treatment with T4 unless the patient was hypothyroid." - excerpt from Life Extention - Thyroid Regulation