Complex sentence: Selenium also plays a role in the functioning of the thyroid gland and in every cell that uses thyroid hormone, by participating as a cofactor for the three known thyroid hormone deiodinases, which activate and then deactivate various thyroid hormones and their metabolites.
Split into simpler sentences: Selenium also plays a role in the functioning of the thyroid gland. These enzymes activate and then deactivate various thyroid hormones and their metabolites.