Iodine deficiency affects almost one billion of people worldwide, and is by far the leading cause of hypothyroidism. Japan is unique in that there are no single patients with hypothyroidism due to nutritional iodine deficiency. Thyroid abnormalities induced by excessive iodine intake, however, is not infrequent in Japan. We report here a case of young woman with severe hypothyroidism. Symptoms such as weight gain, edema, amenorrhea, and constipation were accompanied. She had habitual excessive iodine intake, with the cessation of which her thyroid abnormalities as well as her symptoms subsided. Iodine exerts biphasic effects on the thyroid gland. Its deficiency results in diminished thyroid hormone synthesis ; hence hypothyroidism, since thyroid hormones are iodine-containing amino acids. Its excess inhibits practically all functional aspects of thyroid gland ; iodine uptake from circulation, thyroid hormone synthesis, and its secretion to bloodstream. What makes the matter more perplexing is the fact that the detrimental effect of excess iodine is much more likely to occur in those with pre-existing thyroid disease, even in the mild form. Furthermore, every one or two women out of ten have thyroid diseases such as chronic thyroiditis. Thus supplementation with iodine is by no means encouraged in Japan. Dietitians must be aware of the unique feature of iodine status in Japan
