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European Journal of Epidemiology

, Volume 26, Issue 8, pp 629–635 | Cite as

Doubling in the use of thyroid hormone replacement therapy in Denmark: association to iodization of salt?

  • Charlotte CerqueiraEmail author
  • Nils Knudsen
  • Lars Ovesen
  • Peter Laurberg
  • Hans Perrild
  • Lone Banke Rasmussen
  • Torben Jørgensen
ENDOCRINE EPIDEMIOLOGY

Abstract

Iodization of salt is an effective strategy to prevent iodine deficiency disorders. Recent studies, however, indicate that increasing the iodine intake in a population may give rise to an increased incidence of hypothyroidism, but the association has not been fully clarified. In Denmark, iodization of salt was initiated in 1998 because of mild-to-moderate iodine deficiency. The aim of this study was to evaluate the effect of the raised iodine intake on the nationwide incident use of thyroid hormone replacement therapy (levothyroxine) to treat hypothyroidism. Data on all use of levothyroxine was extracted from the Register of Medicinal Product Statistics during the period 1995–2009 and linked to other nationwide registers by use of the Danish identification number. Persons with previous thyroid surgery were excluded. In the studied period 71,565 incident users were identified. The incidence rate increased 75% in the moderately iodine deficient region (72.2 incident users/100,000 person-years in 1997 to 126.6 in 2008) and 87% in the mildly deficient region (86.9–162.9). When stratified by sex and age-group (00–39, 40–64, 65+) the largest relative increase was seen among women in the youngest age-group, where more than a doubling was seen. The mechanisms behind the increase may be a result of iodine-induced hypothyroidism, although a higher diagnostic activity with regard to thyroid dysfunction and intensified treatment of subclinical hypothyroidism may also play a role. Our findings stress the need for caution when initiating iodine fortification programs to keep the intake within the optimal range, and the need for continuous monitoring.

Keywords

Fortification Hypothyroidism Iodine Pharmacoepidemiology Preventive medicine Thyroid hormone replacement therapy 

Abbreviations

DanThyr

The Danish Investigation of Iodine Intake and Thyroid Diseases.

ATC

Anatomic therapeutical chemical.

DDD

Defined daily dose.

ID

Iodine deficiency.

CI

Confidence interval.

Notes

Acknowledgments

This study was part of The Danish Investigation on Iodine Intake and Thyroid Diseases (DanThyr), which is supported by grants from the Danish Medical Foundation, the 1991 Pharmacy Foundation, North Jutland County Research Foundation, Tømmerhandler Wilhelm Bangs Foundation, Copenhagen Hospital Corporation Research Foundation, The Danish Food Industry Agency, and the Danish Agency for Science Technology and Innovation (grant number 2101-06-0065).

Conflict of interest

The authors declare that no competing financial interests exist.

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Copyright information

© Springer Science+Business Media B.V. 2011

Authors and Affiliations

  • Charlotte Cerqueira
    • 1
    Email author
  • Nils Knudsen
    • 2
  • Lars Ovesen
    • 3
  • Peter Laurberg
    • 4
  • Hans Perrild
    • 2
  • Lone Banke Rasmussen
    • 5
  • Torben Jørgensen
    • 1
    • 6
  1. 1.Research Centre for Prevention and HealthGlostrup University HospitalGlostrupDenmark
  2. 2.Department of Endocrinology and GastroenterologyBispebjerg University HospitalCopenhagenDenmark
  3. 3.Department of GastroenterologySlagelse HospitalSlagelseDenmark
  4. 4.Department of Endocrinology, Aalborg HospitalAarhus University HospitalAalborgDenmark
  5. 5.Department of Nutrition, National Food InstituteTechnical University of DenmarkCopenhagenDenmark
  6. 6.Faculty of Health SciencesUniversity of CopenhagenCopenhagenDenmark

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