Why Simple Goiter is more prevalent in hilly plains and cannot be found in coastal areas?

Simple goiter is caused due to under secretion or hypo secretion of  Thyroxin hormone by Thyroid gland. The mineral iodine is essential for the synthesis of Thyroxin by our thyroid gland. In hilly plains, the iodine content will be washed away through leaching from slopes of the hills during precipitation of rains. Due this vegetables and grains obtained from the crops lack iodine which cause goiter disease. But in coastal regions, the sea food contain iodine in sufficient quantity and so this problem cannot be seen in these places.