Increased risk of dementia in hypothyroidism: A Danish nationwide register-based study.

OBJECTIVE

Globally, the prevalence of individuals with dementia is increasing, and identification of risk factors is of paramount interest. Using population-based registers, we evaluated whether hypothyroidism is a risk factor for dementia.

DESIGN

Register-based cohort study.

PATIENTS AND METHODS

Risk of dementia was evaluated in two cohorts. The DNPR cohort comprises 111,565 hypothyroid patients, diagnosed between 1995 and 2012, and 446,260 euthyroid age- and sex-matched individuals (median follow-up 6.2 years). The OPENTHYRO cohort comprises 233,844 individuals with at least one measurement of serum thyrotropin (TSH) between 1995 and 2011, of whom 2,894 had hypothyroidism (median follow-up 7.2 years). Primary outcome was dementia defined as an International Classification of Diseases 10 code, or prescription of medicine for dementia.

RESULTS

In the DNPR cohort, risk of dementia was significantly increased in subjects with hypothyroidism (HR 1.22; 95% CI: 1.17-1.27), which attenuated after adjusting for pre-existing comorbidity (HR 0.82; 95% CI: 0.79-0.86). Stratification of age into ≤56 and >56 years showed an inverse relationship between age and risk of dementia (HR 2.03; 95% CI: 1.62-2.53 and HR . 1.00; 95% CI: 0.96-1.05). In the OPENTHYRO cohort, the risk of dementia was significantly increased for each 6 months of elevated TSH (HR 1.12; 95% CI: 1.07-1.16).

CONCLUSIONS

Hypothyroidism is associated with increased risk of dementia. The association is influenced by comorbidity and age. Every 6 months of elevated TSH increased the risk of dementia by 12%, suggesting that also the length of hypothyroidism influences the risk of dementia.