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An Increased Incidence of Overt Hypothyroidism after Iodine Fortification of Salt in Denmark: A Prospective Population Study

Department of Endocrinology and Medicine (I.B.P., P.L.), Aalborg Hospital, Aarhus University Hospital, DK-9000 Aalborg, Denmark; Endocrine Unit, Medical Clinic I (N.K., H.P.), Bispebjerg Hospital, DK-2400 Copenhagen, Denmark; Research Centre for Prevention and Health (N.K., T.J.), Glostrup University Hospital, DK-2600 Glostrup, Denmark; The National Heart Foundation (L.O.), DK-1127 Copenhagen, Denmark; and Department of Nutrition (L.B.R.), National Food Institute, DK-2860 Søborg, 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: I.Bulow{at}rn.dk.

Context: Iodine fortification (IF) of salt was introduced in Denmark in 1998. Little is known about the effect of a minor increase in iodine intake on the incidence of hypothyroidism. We prospectively identified all new cases of overt hypothyroidism in two areas of Denmark before and for the first 7 yr after IF had been introduced.

Methods: A computer-based register was used to identify continuously all new cases of overt hypothyroidism in two subcohorts with previous moderate and mild iodine deficiency (ID), respectively (Aalborg, n = 310,124, urinary iodine = 45 µg/liter; and Copenhagen, n = 225,707, urinary iodine = 61 µg/liter). Data were obtained 1) before IF (1997–1998), 2) during voluntary IF (1999–2000), 3) during early (2001–2002) and 4) during late (2003–2005) period with mandatory IF.

Results: The overall incidence rate of hypothyroidism increased during the study period: baseline, 38.3/100,000·yr; voluntary IF, 43.7 (not significant vs. baseline); early mandatory IF, 48.7 [vs. baseline, rate ratio (RR) = 1.27; 95% confidence interval (CI) = 1.10–1.47]; and late mandatory IF, 47.2 (vs. baseline, RR = 1.23; 95% CI = 1.07–1.42). There was a geographic difference because hypothyroidism increased only in the area with previous moderate ID: Aalborg, late mandatory IF vs. baseline, 40.3/29.7 (RR = 1.11; 95% CI = 1.11–1.66); Copenhagen, 56.7/51.6 (RR = 1.10; 95% CI = 0.90–1.34). The increase occurred in young and middle-aged adults.

Conclusion: Even a cautious iodization of salt was accompanied by a moderate increase in the incidence rate of overt hypothyroidism. This occurred primarily in young and middle-aged subjects with previous moderate ID.

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