What causes low white bloodcells (3.6)and low platelets (135)? blood clotting, fasting sugar, thyroid, electrolytes, kidney and liver function are normal.

These are: Lowish but not low enough to do anything but repeat the test. The issue is why were they drawn? Higher WBC counts indicate infection, low counts of one element ( red cell, white cell or platelets) can be a variety of things for each from lack of production to excess destruction, to crowding out by other things. Too much to detail here, repeat the test and call your doctor.