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

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.

Treating Hypothyroidism with Thyroxine/Triiodothyronine Combination Therapy in Denmark: Following Guidelines or Following Trends?

“…Further, this study demonstrated a high risk of overtreatment since 10% had reduced TSH [15]. A register study in Denmark describes an increase of prescriptions of L-T 4 , with an incidence rate increase of 81% over 9 years from 1997 to 2008 [16]. These changes could partly be due to the iodine fortification or a higher diagnostic activity with regard to thyroid dysfunction, but could also be due to an intensified treatment of subclinical hypothyroidism [16].…”

“…A register study in Denmark describes an increase of prescriptions of L-T 4 , with an incidence rate increase of 81% over 9 years from 1997 to 2008 [16]. These changes could partly be due to the iodine fortification or a higher diagnostic activity with regard to thyroid dysfunction, but could also be due to an intensified treatment of subclinical hypothyroidism [16]. In the context of overtreatment, it was striking that we found 28% of the respondents adjust their medicine themselves (probably due to changes in the pattern of symptoms) and 14% of all respondents reported a suppressed TSH <0.01 mU/l.…”

European Thyroid Association (ETA) and Thyroid Federation International (TFI) Joint Position Statement on the Interchangeability of Levothyroxine Products in EU Countries

“…In 2008, there were approximately 80.000 patients in Denmark treated with levothyroxine [3]. Since GSK had a monopoly position, all these subjects were treated with a GSK product, Eltroxin.…”

Changes in the prevalence of hypothyroidism: the HUNT Study in Norway

“…Thus, the annual number of thyroid hormone prescription items dispensed in England more than doubled from 1998 to 2007 (12), and in the Tayside region in Scotland, the prevalence of treated hypothyroidism increased by 63% from 1994 to 2001 (13). In Denmark, the incidence rate of treated hypothyroidism increased by w80% from 1997 to 2008, which may be explained both by more extensive detection and treatment of hypothyroidism and by an increased incidence of hypothyroidism due to the introduction of iodized salt (14). However, the Norwegian population has long been considered to be iodine sufficient (15), and it seems unlikely that changes in iodine intake may explain the changes that we observed.…”