Being lifelong underweight, unable to gain weight while eating more than average/ >500kcal surplus. no celiac disease, diabetes or overactive thyroid. is too fast metabolism possible with normal thyroid function? what else can cause this?

Some basics: The daily caloric need is a factor of existing body mass, physical & mental work. If you are truly consuming more than you need you should gain weight. Celiac disease disease is not the only condition where you can fail to absorb calories. A sit down with a nutritionist to chart actual intake as well as more testing would be needed. Over & underestimating needs/consumption is a factor.