Hypothyroidism is associated with an increased risk for dementia, according to the results of a nationwide study published in Clinical Endocrinology.
Though some studies have shown an association between hypothyroidism and dementia, the results have been conflicting, possibly due to a lack of statistical power or to inconsistent definitions of hypothyroidism and dementia.
To investigate the potential association of hypothyroidism with an increased risk for dementia, longitudinal data from 275,467 individuals (75% women) with at least one serum thyrotropin (TSH) measurement were analyzed. Hypothyroidism was defined as having had at least two prescriptions for thyroid hormone dispensed, or having at least 2 consecutive elevated TSH values (>4.0mIU/L) and confirmed with a hypothyroidism ICD-10 code. Dementia was defined as having had a relevant ICD-10 or ATC code recorded.
A higher prevalence of dementia was observed among individuals with hypothyroidism than those without (P <.001). Risk for dementia over time was significantly higher in individuals with hypothyroidism than those without (hazard ratio [HR], 1.22; 95% CI, 1.17-1.27). This association was greatly diminished when controlling for pre-existing morbidity. The association was not diminished when stratifying individuals into age groups that were ≤56 and >56 years. Younger individuals with hypothyroidism had a significantly greater risk for dementia than their eurothyroid counterparts, even after controlling for comorbidities (HR, 2.03; 95% CI, 1.62-2.53). Individuals with hypothyroidism had a significantly greater risk for dementia for each 6 months of elevated TSH (HR, 1.12; 95% CI, 1.07-1.16).
The results of this study indicated that hypothyroidism was significantly associated with an increased risk for dementia, which was influenced by pre-existing morbidity and age. A 12% increased risk for dementia for each 6 months of elevated TSH was observed, which may be explained by the role of thyroid hormones in the regulation of amyloid-β.
Limitations to this study include the inability to determine the severity, cause, or treatment of hypothyroidism experienced by included individuals. The progressive nature of both hypothyroidism as well as dementia make it difficult to determine disease onset. The use of register-based data in the definition of the conditions leads to vulnerability to physician misclassification.
Future research investigating the association of hypothyroidism treatments with the risk of dementia is warranted.
Thvilum M, Brandt F, Lillevang-Johansen M, Folkestad L, Brix TH, Hegedüs L. Increased risk of dementia in hypothyroidism. A Danish nationwide register-based study. [published online January 22, 2021]. Clin Endocrinol (Oxf). doi:10.1111/cen.14424