Heredity. It is quite possible that thyroid problems develop more frequently in humans whose ancestors had any throat diseases or thyroid disease itself. In fact, one cannot claim that thyroid disease is pure heredity case.
Moreover, it is not the main reason of thyroid problems. Nevertheless, the hereditary predisposition to this kind of health problems exists. People, whose relatives or ancestors have some medical thyroid problems, should be more careful with appearance of suspicious thyroid symptoms.