Both low and high iodine intake levels associate with increased occurrence of disease. The iodine intake level recommended by the World Health Organisation is 150 mcg/day for adults. The prevalence of hyperthyroidism in the later ages of life increases up to 2 percent for women, toxic multinodular goitre (TMNG) being the most frequent etiology. The thyroid gland of human adults secretes approximately 80 mcg of thyroxine per day, corresponding to 52 mcg iodine, an amount of iodine the gland must take up daily in order to remain in balance. Intakes of up to 600 mcg per day in the European Union and 1100 mcg per day in the United States are declared tolerable for adults. Therefore, higher values are, by definition, excessive, yet the above cut-off rate is arbitrary, since most individuals tolerate higher intakes, while a few already have untoward effects at lower intakes. According to Burgi (2010), even modest increases in iodine supply can trigger hyperthyroidism in some individuals, including iodine intakes below 300 mcg per day.
Burgi, H. 2010. Iodine excess. Best Practice & Research Clinical Endocrinology & Metabolism, 24, 107-115.