What causes a thyroid to become underactive?

Various causes: Most common cause is the body's own immune system attacks it and destroys it (or at least enough of it so that normal levels of thyroid hormone can't be made & secreted). This is called autoimmune hypothyroidism (aka Hashimoto's). Other causes: a surgeon removes the thyroid, the thyroid is destroyed by a dose of radioactive iodine used to treat hyperthyroidism.