why goiter is very common in north india ?

Goiter is a disease of the thyroid gland in which it becomes enlarged. Iodine is a necessary constituent of a person's diet. It is a component of Tri and tetraiodothyronine, the precursor molecules from which thyroxine is made.  As the people living in south live near sea and consume more sea food which is rich in iodine than people living in north. This is the reason that goiter is very common in north India.