Simple goitre is usually seen in people living in the hilly regions. Give two reasons for the statement.

1. The soil of hilly regions is POOR IN IODINE, because rain and melting snow wash the soluble iodine salts down the slopes into the plains and the sea. The crops grown there, and so the diet of the people, are therefore deficient in iodine.
2. Without iodine the thyroid cannot make THYROXINE. The pituitary responds by pouring out more TSH, which makes the thyroid gland work harder and ENLARGE — producing the swelling in the neck called simple goitre.

Two marks: the ENVIRONMENTAL reason and the PHYSIOLOGICAL reason.

The first explains why the deficiency exists in that particular place — leaching of iodine from hill soils by rain and snow melt. The second explains why the deficiency produces a SWELLING rather than simply a shortage.

THE MECHANISM IS THE INTERESTING PART: the gland enlarges not because it has too much iodine but because it has too little. Lacking iodine, it cannot make thyroxine; the low thyroxine level removes the feedback that normally restrains the pituitary; the pituitary secretes more TSH; and TSH stimulates the thyroid tissue to grow. The gland grows larger while producing less.

This is why table salt is IODISED — a cheap public-health measure that has largely eliminated the disease in India.