The thyroid gland secretes hormones that set the speed of your metabolism. If you’re cold, the thyroid gland speeds up your metabolism to warm you up. If you’re hot, it slows down your metabolism to cool you off. With hypothyroidism, the gland fails to secrete enough thyroid hormones and the body can’t adapt efficiently to these kinds of fluctuations, whether external (temperature) or internal (stress). Every cell in the body and brain uses thyroid hormones, and when they don’t get enough the cells lose energy and function poorly. As a result, energy production in the body and brain plummet, leaving you tired and mentally foggy.