Hypothyroidism, also known as an underactive thyroid, is a condition where your thyroid gland doesn't produce enough thyroid hormones. This can slow down your body's metabolism, leading to a wide range of symptoms. Common symptoms of hypothyroidism include: Profound fatigue and lack of energy; increased sensitivity to cold and feeling chilly; dry skin and hair loss; unexplained weight gain; muscle and joint aches with weakness; constipation; hoarseness or a deep voice; elevated cholesterol levels; slow heart rate (bradycardia); depression and slowed cognitive function (e.g., memory problems); irregular menstrual cycles and fertility issues in women; hearing problems.