A goiter may form because __________?
1) none of the listed responses is correct
2) there is too much iodine in the diet
3) the thyroid cannot release T3 and T4 so the hormones build up in the gland
4) low blood levels of T3 and T4 inhibit the negative feedback loop, and an overload of TSH causes enlargement of the gland
5) low blood levels of T3 and T4 inhibit the negative feedback loop, and an overload of TRH causes enlargement of the gland