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

Untreated hypothyroidism is common in iodine-replete areas. Frequent thyroid function testing and use of levothyroxine treatment for subclinical hypothyroidism suggest that the prevalence may have decreased. Therefore, in this study, we examined changes in the prevalence of hypothyroidism in a Norwegian county from 1995-1997 to 2006-2008.

Population surveys of 33,917 individuals in 1995-1997 and 49,180 individuals in 2006-2008 were carried out.

We compared the prevalence of untreated overt, untreated subclinical, and treated hypothyroidism between 1995-1997 and 2006-2008.

The prevalence of untreated overt hypothyroidism among women decreased by 84% from 1995-1997 (0.75%) to 2006-2008 (0.12%) (prevalence ratio (PR) 0.16; 95% CI 0.10-0.26). The corresponding decrease among men was 43% from 0.21 to 0.12% (PR 0.57; 95% CI 0.28-1.16). The prevalence of untreated subclinical hypothyroidism decreased by 64% from 3.0 to 1.1% in women (PR 0.36; 95% CI 0.31-0.42) and decreased by 54% from 2.1 to 1.0% in men (PR 0.46; 95% CI 0.38-0.56). Conversely, the prevalence of treated hypothyroidism among women increased by 60% from 5.0 to 8.0% (PR 1.60, 95% CI 1.50-1.71), and the corresponding prevalence in men doubled from 1.0 to 2.0% (PR 1.96; 95% CI 1.59-2.41). The prevalence of any form of hypothyroidism remained essentially similar at 9% in women and 3% in men.

The prevalence of untreated hypothyroidism in this Norwegian county decreased strongly from 1995-1997 to 2006-2008. The findings suggest that the prevalence of untreated hypothyroidism in populations with easy access to thyroid function testing and levothyroxine treatment may now be low.