More than adequate or excessive iodine intake may lead to hypothyroidism and autoimmune thyroiditis.

Iodine nutrition is an important factor associated with TSH concentration even in the rigorously selected reference population. Baseline TSH of 1.0-1.9 mIU/l is an optimal interval with the lowest incidence of abnormal TSH in 5 years.

The relationship between iodine and the risk for the occurrence of diffuse goiter shows a U-shaped curve. Nodular goiters are more prevalent in iodine-deficient areas.

Objective: To investigate the effect of different levels of iodine intake on the prevalence of hyperthyroidism and the impact of universal salt iodization on the incidence of hyperthyroidism. Design: A comparative cross-sectional and longitudinal survey was conducted in three areas with borderline iodine deficiency, mild iodine excess (previously mild iodine deficiency) and severe iodine excess. Universal salt iodization had been introduced 3 years previously except in the area with borderline iodine deficiency. Methods: In total 16 287 inhabitants from three areas answered a questionnaire concerning the history of thyroid disease. Among them 3761 unselected subjects received further investigations including thyroid function, thyroid autoantibodies, thyroid ultrasonography and urinary iodine excretion. Results: Among areas with median urinary iodine excretion of 103 mg/l, 375 mg/l and 615 mg/l ðP , 0:05Þ; the prevalence of hyperthyroidism did not differ significantly (1.6%, 2% and 1.2%). The prevalence of subclinical hyperthyroidism was higher in areas with borderline iodine deficiency and mild iodine excess than in the area with severe excess iodine intake (3.7%, 3.9% and 1.1%, P , 0:001). The prevalence of Graves' disease and its proportion in hyperthyroidism did not differ among areas. The incidence of hyperthyroidism did not significantly increase after the introduction of universal salt iodization. Conclusion: Different iodine intakes under a certain range do not affect the prevalence and type of hyperthyroidism. Subclinical hyperthyroidism is more prevalent in the iodine deficient area than in the severe iodine excessive area. In the area with mild iodine deficiency, the introduction of universal salt iodization may not be accompanied by an increased incidence of hyperthyroidism.