Hypothyroidism occurs when the thyroid gland does not produce enough thyroid hormone.
The thyroid gland is a bowtie-shaped organ in your neck, below the larynx. It secretes hormones that control how fast your heart beats, how quickly you digest food, how much you sweat, the speed at which you burn calories and many other activities, according to the American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS).
The most common cause of hypothyroidism is the body's immune reaction to itself, producing antibodies against the thyroid gland. One such autoimmune disorder is called Hashimoto's thyroiditis, an inflammation of the thyroid gland. Other causes include treatment of hyperthyroidism such as radioactive iodine treatment or surgery.
Researchers have not discovered why the immune system mistakes the thyroid for a harmful invader, such as a virus. It has been suggested that many factors lead to the disorder, including age, heredity and gender. This is because the condition is most common among middle-aged women. It also is common among biological family members.