Underactive Thyroid Gland is called as Hypothyroidism. It is a term used to describe a condition in which there is a reduced level of Thyroid hormone (thyroxine) in the body. This can cause various symptoms, the most common being: tiredness, weight gain, constipation, aches, dry skin, lifeless hair and feeling cold. Thyroxine is a hormone (body chemical) made by the thyroid gland in the neck. It is carried round the body in the bloodstream. It helps to keep the body’s functions (the metabolism) working at the correct pace. Many cells and tissues in the body need thyroxine to keep them going correctly. When thyroid gland is unable to make enough thyroxine it results in hypothyroidism. Hypothyroidism causes many of the body’s functions to slow down. Hypothyroidism may also occur if there is not enough thyroid gland left to make thyroxine, eg after surgical removal or injury. (In contrast, if you have hyperthyroidism, you make too much thyroxine. This causes many of the body’s functions to speed up.)