When you have a deficiency of Selenium it can cause symptoms of hypothyroidism, including extreme fatigue, mental slowing, goiter, cretinism, and even miscarriage especially reoccurring miscarriages.
A great source of Selenium are Brazil nuts and the selenium contained in Brazil nuts is also an antioxidant so it's good for your overall health to eat Brazil nuts especially if you have a selenium deficiency.
Selenium is necessary for the conversion of the thyroid hormone thyroxine.