Goiter prevalence and iodine metabolism were studied in areas of endemic goiter in the Himalayas in India and Nepal. Similar studies were also made in Ceylon. The results are compatible with the hypothesis that severe environmental deficiency of iodide is the primary factor responsible for endemic goiter in these areas. The endemicity was less severe in Ceylon than in India and Nepal. The thyroid glands of persons living in endemic areas show an interesting adaptive response to maintain internal homeostasis in the face of severe iodine deficiency. The mechanism of this adaptation was studied in thyroids of goats raised in endemic and nonendemic areas. Thyroids of goats living in an area of severe iodine deficiency showed higher MIT/DIT and T3/T4 ratios than glands of those in an area of iodine abundance. There was a higher incorporation of 131I in 27S iodoproteins in the iodide-deficient glands. A decrease in iodine concentration of the thyroid and an increase in circulating TSH levels are possibly involved in mediating this response but of the two, the former mechanism seems more likely than the latter
