Hypothyroidism and hyperthyroidism are both thyroid conditions caused by an imbalance of thyroid hormone production. Hypothyroidism means an underactive thyroid, while hyperthyroidism means an overactive thyroid. Symptoms of hypothyroidism Fatigue Weight gain Cold intolerance Dry skin and hair Constipation Depression Menstrual irregularities Slowed heart rate Enlarged thyroid, also called a goiter Symptoms of hyperthyroidism Weight loss Nervousness Frequent bowel movements Sensitivity to cold Muscle weakness Rapid heart rate (tachycardia) and/or heart palpitations Increased blood pressure Shakiness, like hand tremors Feeling anxious, nervous and/or irritable