In addition to Hashimoto thyroiditis, lupus can cause both hives and tachycardia. The blood tests for all the possible causes are numerous and expensive. Hopefully the doctors recommended in your area can see you soon and insurance will cover the cost of all the tests. If you want to do your own tests, I can make some suggestions of what to include and you can get the prices from the lab before they are drawn. Have you gotten a definite diagnosis of mast cell activation syndrome or has this just been mentioned as a possibility? Hives and the itching are miserable! I hope you get help soon!