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 prior to and for the first 7 years after IF had been introduced.

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

Results: The overall incidence rate of hypothyroidism increased during the study period (baseline: 38.3/100,000/year; voluntary IF: 43.7 (vs. baseline: ns); early mandatory IF: 48.7 (vs. baseline: rate ratio (RR) (95% confidence interval (CI)): 1.27 (1.10-1.47)); late mandatory IF: 47.2 (vs. baseline: RR (CI): 1.23 (1.07-1.42)). There was a geographic difference as hypothyroidism increased only in the area with previous moderate ID (Aalborg, late mandatory IF vs. baseline: 40.3/29.7, RR (CI): 1.35 (1.11-1.66); Copenhagen: 56.7/51.6, RR (CI): 1.10 (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.