No external cause can be identified in 80 to 90 percent of adults and children with chronic urticaria.
Yet you should try to identify any possible allergen. Hope your doctor might have advised you regarding keeping an allergen diary
Urticaria can be triggered by a variety of physical factors
1. Exposure to cold (the hives often appear as the cold-exposed skin rewarms
2. Changes in body temperature/sweating (these hives are often tiny and numerous and appear on reddened skin)
3. Vibration (palms may become red, swollen, and itchy after holding onto the steering wheel of a car while driving)
4. Pressure (hives on the palms or the soles of the feet can occur hours after carrying heavy objects or walking long distances.
5. Because the skin on the palms and soles is thick, these areas may appear reddened and swollen without clear hives)
6. Exercise (hives that appear during exercise can be a sign of dangerous condition called exercise-induced anaphylaxis,
7. Sunlight or water
You can consider getting an allergen test and thyroid test done.
Treatment includes antihistamines (including new generation ones), and in resistant cases oral steroids, cyclosporine, doxepine etc. Please consult you doctor for more details