Hypothyroidism (or underactive thyroid)

• When your thyroid gland doesn't make enough thyroid hormone
• May cause weight gain, constipation, slowed heart rate, dry skin and hair, sensitivity to cold

Hyperthyroidism (or overactive thyroid)

• When your thyroid gland makes too much thyroid hormone
• May cause weight loss, nervousness, frequent bowel movements, sensitivity to cold, and muscle weakness

Both

• More likely to affect women
• Symptoms like fatigue and an enlarged thyroid, also called a goiter, which may make your neck look swollen
• Can be caused by autoimmune diseases, which cause your body to attack itself