One of the most common causes of neck swelling is disease of the thyroid gland. So, what is thyroid gland? What happen when it’s malfunctioning? Well, thyroid gland is a large ductless gland that lies at the lower front of the neck, secreting thyroid hormones; thyroxine (T4) and triiodothyronine (T3); directly into the bloodstream. These hormones affect almost all the cells in the body. They control much of the metabolism and stimulate the growth of child.