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Increase in Incidence of Hyperthyroidism Predominantly Occurs in Young People after Iodine Fortification of Salt in Denmark

Department of Endocrinology and Medicine (I.B.P., P.L.), Aalborg Hospital, Aarhus University Hospital, 9100 Aalborg, Denmark; Endocrine Unit (N.K., H.P.), Medical Clinic I, Bispebjerg Hospital, 2400 Copenhagen, Denmark; Research Centre for Prevention and Health (N.K., T.J.), Glostrup University Hospital, 2600 Glostrop, Denmark; The National Heart Foundation (L.O.), 1127 Copenhagen, Denmark; and Department of Nutrition (L.B.R.), Danish Institute for Food and Veterinary Research, 1790 Copenhagen, Denmark

Address all correspondence and requests for reprints to: Inge Bülow Pedersen, Department of Endocrinology and Medicine, Aalborg Hospital, Aarhus University Hospital, DK-9000 Aalborg, Denmark. E-mail: Ibulow{at}aas.nja.dk.

Context: To prevent goiter 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: We used a computer-based register of all new cases of hyperthyroidism in two population subcohorts with moderate iodine deficiency (Aalborg, n = 310,124) and mild iodine deficiency (Copenhagen, n = 225,707), respectively. Data were obtained 1) before IF (1997–1998); 2) during voluntary IF (1999–2000); 3) during the early (2001–2002) period of mandatory IF; and 4) during the 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, 20–39, 40–59, and 60+ yr (P for trend <0.001). The increase was relatively highest in young adults aged 20–39 yr: late mandatory IF (percent increase from baseline), age 20–39, 160%, P < 0.001; age 40–59, 29%, P < 0.01; age 60+ yr, 13%, P = not significant.

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. Furthermore, monitoring is expected to show a decrease in the number of elderly subjects suffering from nodular hyperthyroidism.

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