Thyroid Disorders Explained | Hyperthyroidism vs Hypothyroidism in 20 Seconds Your thyroid gland is a small, butterfly-shaped organ in the neck, but it controls some of the body’s most important functions. If it works too much, it leads to hyperthyroidism, causing weight loss, fast heartbeat, and sweating. If it works too little, it leads to hypothyroidism, causing fatigue, weight gain, and feeling cold. This short 3D medical video explains thyroid disorders, their symptoms, diagnosis, and treat