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Submitted on March 24, 2006
Accepted on July 11, 2006
Department of Endocrinology and Medicine, Aalborg Hospital, Aarhus University Hospital; Endocrine Unit, Medical Clinic I, Bispebjerg Hospital; Research Centre for Prevention and Health, Glostrup University Hospital; The National Heart Foundation, Copenhagen; Department of Nutrition, Danish Institute for Food and Veterinary Research
* To whom correspondence should be addressed. E-mail: Ibulow{at}aas.nja.dk.
To prevent goitre and nodular hyperthyroidism iodine fortification (IF) of salt was introduced in Denmark in 1998. We prospectively registered all new cases of overt hyperthyroidism in two areas of Denmark before and for the first 6 yr after iodine fortification.
Methods: A computer based register of all new cases of hyperthyroidism in two population sub-cohorts with moderate iodine deficiency (ID) (Aalborg, n = 310,124) and mild ID (Copenhagen, n = 225,707), respectively. Data were obtained: 1. before IF (1997-1998); 2. during voluntary IF (1999-2000); 3. during early (2001-2002); and 4. late (2003-2004) period with mandatory IF.
Results: The overall incidence rate of hyperthyroidism increased (baseline: 102.8/100,000/year; voluntary IF: 122.8; early mandatory IF: 140.7; late mandatory IF: 138.7 (p for trend < 0.001)). Hyperthyroidism increased in both sexes (P < 0.001) and in all age groups: 0-19 yr, 20-39, 40-59 and 60+ (p for trend < 0.001). The increase was relatively highest in young adults aged 20-39 yr; late mandatory IF (per cent increase from baseline): age 20-39: 160%, P < 0.001; age 40-59: 29%, P < 0.01; age 60+ years: 13%, p = ns.
Conclusion: Even a cautious iodization of salt results in an increase in the incidence rate of hyperthyroidism. Contrary to current concepts many of the new cases were observed in young subjects, and are presumably of autoimmune origin. Further monitoring is expected to show a decrease in the number of elderly subjects suffering from nodular hyperthyroidism.
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