Just wandering if you get tolerant to vitamins and how do you know if your absorbing them?

Do you get tolerant to MB12? Methylcobalamin? Its a b vitamin? I know Picamilon is a B vitamin for anxiety and it works but it stops after a while.

Why is this I always worry things stop working or if you cant absorb them or if the brands not good quality?

ALso is a good thyroid test done through blood?