The ultimate sulfur molecule is glutathione, and its function in life is to combat oxidative stress. In reduced form it is a reservoir for electrons and the most potent antioxidant we make. Selenium is an ideal carrier for electrons. It picks them up easily but just as easily gets rid of them.
We must have enough selenium from the diet to combat oxidative stress. Some soils are famous for being extremely selenium-deficient and resulted in higher rates of hypothyroidism, goiter, cretinism, miscarriages, and extreme fatigue. A selenium deficiency state can be evident as fatigue and impaired cognitive function, as well as thyroid dysfunction.
Mercury binds to a form of selenium called selenocysteine. It is the regular cysteine molecule, but the sulfur element has been replaced by selenium. The affinity of mercury for that molecule is 10 to the 45th power. Unfortunately, that is actually a million-fold higher affinity than for the glutathione that would normally bind to that molecule. Mercury can bind so tightly to selenoproteins that a normal diet is not going to meet the body’s demands.