The thyroid gland is located at the bottom of the neck. The main job of the thyroid gland is to control our metabolism. Our metabolism is controlled by two hormones, thyroxine (known as T4) and triiodothyronine (T3). If your thyroid gland is less active, then it also produces fewer hormones and this leads to a condition called hypothyroidism. People with hypothyroidism use energy more slowly and their metabolism is also slowed down (weight gain, drowsiness, fatigue, etc.). People with hyperthyroidism produce too many thyroid hormones which increase the body's energy expenditure. Excess thyroid hormones often result in a significant weight loss (5 - 10 kg, sometimes more, other symptoms: sweating, insomnia, palpitations).