5. What causes most thyroid disease in the US?
From Quiz Thyroid Physiology
Answer:
Auto-immunity
Auto-immunity, in which the immune system mistakes one or more thyroid proteins for foreign proteins, such as those associated with bacteria or viruses, is the main cause of thyroid disease in the US. Auto-immunity can cause the thyroid to be underactive (hypothyroid), or overactive (hyperthyroid). It can cause widespread enlargement resulting in a goiter or regional enlargement leading to thyroid nodules.
Iodine deficiency is almost unheard of in the US because most salt is fortified with sodium iodide.
Thyroid cancer does occur, but is much less common than breast, lung, or colon cancer, and can be successfully treated in a majority of cases.
Radiation exposure can cause thyroid disease, but is also uncommon. In regions downwind from Chernobyl, however, rates of hypothyroidism, and both cancerous and benign thyroid nodules occured at much higher rates than usual for many years after the accident.