Too little hormone, as in hypothyroidism, may cause you to gain weight. Also, if you don't have enough hormone, your heartbeat slows down and your body temperature decreases. In addition, food moves through your intestines more slowly and your muscles contract more slowly. It is a disorder of your immune system. The immune system normally protects you from infection.
Radioactivity often destroys the gland, so that the body needs replacement thyroid hormone.
People who have received x-ray treatment for cancers of the head and neck may develop hypothyroidism, if their thyroid was exposed to radiation. They can infect the thyroid gland and cause it to produce too little hormone. These infections can be treated and do not usually cause permanent hypothyroidism. The pituitary gland stimulates the thyroid to produce hormones. The pituitary may fail to stimulate the thyroid to make enough hormones to meet your body's needs.