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J Clin Endocrinol Metab. 2018 Dec 14. doi: 10.1210/jc.2018-01993. [Epub ahead of print]

Increased incidence rate of hypothyroidism after iodine fortification in Denmark. A 20 year prospective population-based study.

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Department of Endocrinology, Aalborg University Hospital, Denmark.
Department of Endocrinology, Bispebjerg Hospital, University of Copenhagen, Denmark.
Department of Geriatrics, Aalborg University Hospital, Denmark.
Centre for Clinical Research and Prevention, Bispebjerg/Frederiksberg Hospital, Denmark.
Department of Public Health, Faculty of Health Sciences, University of Copenhagen.
Faculty of Medicine, Aalborg University.
Department of Gastroenterology, Slagelse Hospital.



To monitor the impact of a cautious iodine fortification (IF) on the incidence of overt hypothyroidism in two sub-populations with different levels of preexisting iodine deficiency (ID).


A 20 years (1997-2016) prospective population-based study identified all new cases of diagnosed overt biochemical hypothyroidism in two open cohorts: a Western cohort with moderate ID (n=309,434, January 1st, 1997) and an Eastern cohort with mild ID (n=224,535, January 1st, 1997). A diagnostic algorithm was applied to all thyroid function tests performed within the study areas and possible new cases were verified individually. Mandatory IF of salt was initiated in mid-2000 (13 p.p.m.). The present study is a part of the DanThyr study.


At baseline, standardized incidence rate (SIR) of hypothyroidism was 32.9 and 47.3/100.000/year in the cohort with moderate and mild ID, respectively. The SIR of hypothyroidism increased significantly in both cohorts after implementing mandatory IF with peak values of 150 % in 2014-16 for the moderate ID cohort and 130 % in 2004-05 for the mild ID cohort. Significant increases in SIR were seen among the young and middle aged of both cohorts, whereas no changes were seen among the elderly (60+ y). The follow-up period for the mildly iodine deficient cohort was restricted up to and including 2008.


The cautious initiation of the IF program in Denmark caused a sustained increase in hypothyroidism incidence among subjects residing areas of moderate and mild iodine deficiency, but only among the young and middle aged.


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