INCREASED INCIDENCE OF THYROTOXICOSIS AFTER IODINE SUPPLEMENTATION IN AN IODINE SUFFICIENT AREA

The aim of this study was to determine the incidence rate of thyrotoxicosis (TT) before and during dietary-iodine supplementation in an iodine-sufficient area. Population: the study was carried out in Vigo, South Galicia (northwest of Spain), from January 1977 to December 1989. The mean population throughout the study period was 267,330 inhabitants (47% males and 53% females). From January 1985, a mandatory consumption of iodinized salt on the whole population started in Galicia. This region was considered as an iodine-deficient area but Vigo is an iodine-sufficient area. Measurements: all newly diagnosed TT cases in Vigo city within the study period were included in this study. Diagnosis of TT was based on the clinical manifestations, an elevated level of T-4 and suppressed TSH values. The difference between diffuse and nodular forms of goiter was assessed by scintigraphy. The average inidence rate (AIR) was determined in two periods, before (period A, 1977-1984) and during (period B, 1985-1989) iodine supplementation. Results: the AIR throughout the whole study period was 4.89 new cases per 100,000 population, 95% confidence limits 4.16 to 5.63; AIR was 1.34 for males and 8.03 for females. AIR in period B was significantly higher with respect to period A, 7.68 and 3.10 per 100,000 respectively. The confidence interval for the difference was from -20.4 to 30.1, p<0.05. The increase of the incidence of TT was comprised of both nodular and diffuse goiters. Conclusions: dietary iodine supplementation in iodine-sufficient areas may induce an increase of the incidence of TT. This could be explained by the fact that individuals and populations differ widely in terms of optimal daily requirements as well as adverse responses to both deficiency and excess of iodine.