The thyroid gland controls major body functions related to the brain and heart. An overactive thyroid causes hyperthyroidism with symptoms like mood swings, irregular periods, weight loss and fatigue. An underactive thyroid results in hypothyroidism with symptoms such as depression, constipation, weight gain and fatigue. Some warning signs of potential thyroid problems include unexplained mood swings, changes in menstrual periods, bowel issues, weight fluctuations, fatigue, a family history of thyroid disorders, cholesterol problems, neck swelling, dull hair and skin, and constant body aches. Any combination of these symptoms could indicate an underlying thyroid problem that requires medical attention.