What Causes Persistent Fatigue Along With Change In Body Temperature?

I have a cousin that has been sick for about 2 months. He has been to the doctor numerous times. they have run test but can not find nothing. He is 34 year old and he feels tired all the time, can be clammy one minute cold the next? His seems to have some come kind of allergies (sinus??) Does it sound like a thryroid issue? What could cause this?