I've seen this question posted several times so I went researching. I have had an itchy problem for several years. Just this week, I left a large bruise on my thigh from scratching in my sleep, so this is somewhat important to me. I hope this helps answer some questions.
Found this on thyroid.about.com
http://thyroid.about.com/cs/relatedconditions1/l/blurticaria.htm
"The researchers also believe that for some patients with chronic urticaria, there is a link to autoimmune disease, and in particular, thyroid disease, as a significant percentage of patients with urticaria also have high levels of antithyroid antibody, even though they may not have clinical thyroid disease."
And this from http://autoimmunedisease.suite101.com/article.cfm/rashes_and_hives:
"The Role of Thyroid Antibodies
Several groups of researchers have described a relationship between chronic urticaria and thyroid disease, primarily Hashimoto’s thyroiditis and to a lesser extent Graves’ disease. These patients typically have levels thyroid peroxidase antibodies. In general, these patients had severe chronic urticaria/angioedema and no other signs of thyroid disease although some patients had a slight goiter. Treatment with levothyroxine caused improvement in the urticaria and angioedema. When treatment was stopped the symptoms returned and then subsided with the continuation of treatment. In the patients without evidence of thyroid antibodies, levothyroxine had no effect. The pathogenic relationship between thyroid antibodies and the chronic urticaria hasn’t yet been determined."