The rise in thyrotoxicosis incidence with cautious mandatory IF returned to baseline level after 7-8 years and levelled out at 33% below baseline in the population with previously moderate ID after 16-17 years.

Objective To investigate the impact of mandatory iodine fortification (IF) on the incidence of nosological subtypes of overt thyrotoxicosis and hypothyroidism. Design We identified and scrutinized all possible new cases of overt thyrotoxicosis and hypothyroidism in an open cohort in Northern Jutland (n = 309 434; 1 January 1997) during the years 2014‐2016. Individual medical history was evaluated to verify and detail the incidence of overt thyroid dysfunction and for classification into nosological subtypes. A number of cases were excluded during final verification due to spontaneous normalization of thyroid function, as they had no medical history suggesting a known condition, which could transiently affect thyroid function (subacute/silent thyroiditis, PPTD and iatrogenic thyroid dysfunction). An identical survey was conducted in 1997‐2000 prior to mandatory IF of salt (13 µg/g) that was in effect from year 2001. Results The standardized incidence rate (SIR) of verified overt thyrotoxicosis decreased markedly from 97.5/100 000/year in 1997‐2000 to 48.8 in 2014‐2016 (SIRR: 0.50 [95% CI: 0.45‐0.56]). This was due to a distinct decrease in the SIR of multinodular toxic goitre (SIRR: 0.18 [0.15‐0.23]), solitary toxic adenoma (SIRR: 0.26 [0.16‐0.43]) and to a lesser degree Graves’ disease (SIRR: 0.67 [0.56‐0.79]). SIR for overt hypothyroidism was unaltered by 2014‐2016 (SIRR: 1.03 [0.87‐1.22]). However, age distribution shifted with more young and fewer elderly cases of verified overt hypothyroidism. Conclusion Mandatory IF caused a substantial reduction in SIR of verified overt thyrotoxicosis (especially of nodular origin) while avoiding an increase in SIR of verified overt hypothyroidism.

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