Hypothyroidism, also called an underactive thyroid, happens when the thyroid gland does not make enough thyroid hormone for the body’s needs. Because thyroid hormone helps regulate energy use throughout the body, low thyroid levels can affect energy, weight, mood, digestion, skin, hair, heart rate, menstrual cycles, and temperature tolerance. Hypothyroidism can develop slowly, and symptoms may be easy to miss or mistake for stress, aging, poor sleep, depression, menopause, or