Did any of the doctors check to see if your thyrhoid was ok? It can be checked with a simple blod test. Are you taking a good multi vitamin also? And your diet can affect your nails....
Could be an underlying problem like llike liver. But it could be a fungus also. Try Rinsing your nails in lemon juice every day.
Find another Doctore that will test your blood and see what is amiss.
