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Effect of a Mandatory Iodization Program on Thyroid Gland Volume Based on Individuals’ Age, Gender, and Preceding Severity of Dietary Iodine Deficiency: A Prospective, Population-Based Study

Department of Internal Medicine I (P.V., N.K., H.P.), Bispebjerg Hospital, University of Copenhagen, 2400 Copenhagen, Denmark; Department of Endocrinology (A.C., P.L., I.B.P.), Aalborg Hospital, University of Aarhus, 9000 Aalborg, Denmark; Department of Nutrition (L.B.R.), Danish Institute for Food and Veterinary Research, 2860 Søborg, Denmark; Danish Heart Association (L.O.), 1127 Copenhagen, Denmark; and Research Centre for Prevention and Health (P.V., N.K., T.J.), 2600 Glostrup, Denmark

Address all correspondence and requests for reprints to: Pernille Vejbjerg, Department of Internal Medicine, Bispebjerg Hospital, Bispebjerg Bakke 23, 2400 Copenhagen NV, Denmark. E-mail: pervej01{at}glostruphosp.kbhamt.dk.

Objective: We aimed to evaluate prospectively the effect of 4 yr of mandatory iodization of salt (13 ppm iodine) on thyroid volume in two regional areas with respectively mild and moderate iodine deficiency.

Methods: Two separate cross-sectional studies were performed before (n = 4649) and after (n = 3570) the iodization in year 2000 in two areas with mild and moderate iodine deficiency. Women aged 18–22, 25–30, 40–45, and 60–65 yr and men aged 60–65 yr were examined. Thyroid ultrasonography was performed.

Results: A lower median thyroid volume was seen in all age groups after iodization. The largest relative decline was found among the younger females from the area with previous, moderate iodine deficiency. Only a minor decrease was seen among the youngest participants in the area with previous, mild iodine deficiency. After iodization, there were no regional differences in median thyroid volume in the age groups younger than 45 yr. When adjusted for confounders, a lower mean volume was seen among those with multiple nodules in both areas and in the group with diffuse structure in the area with moderate iodine deficiency. Before the iodization, 17.6% of the total cross-section had thyroid enlargement; after the iodization, 10.9% of the cross-section had thyroid enlargement.

Conclusion: In this prospective study, we demonstrated a lower thyroid volume in all age groups after iodization of salt. The decline was largest in the area with former, moderate iodine deficiency. The equal volumes in the regions among the younger age groups indicate approximation to an optimal iodine intake.

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