l thyroid gland
The human thyroid gland has the highest selenium content per gram of tissue among all organs.The iodothyronine deiodinases (DIO1,2,3), are selenoproteins. Glutathione peroxidase and thioredoxin reductase are selenoproteins that are involved in protection of the thyroid from hydrogen peroxide.
The first step in the synthesis of thyroid hormones is catalyzed by thyroid peroxidase which requires hydrogen peroxide. Hydrogen peroxide used by thyroid peroxidase is produced by dual oxidase 1 and dual oxidase 2. Hydrogen peroxide is both required for thyroid hormone synthesis but also can be toxic to the thyroid gland. Selenium is required both for thyroid hormone synthesis and protection of the thyroid from hydrogen peroxide needed to synthesize thyroid hormones. .