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, the rate at which food moves through your intestines and the rate at which your muscles contract slow down. It is a disorder of your immune system. The immune system normally protects you from infection.
Radioactivity can destroy the gland.
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 may fail to stimulate the thyroid to make enough hormones to meet your body's needs. Some people are born without thyroid glands or with glands that cannot make thyroid hormone.