Levothyroxine treatment and incident dementia in adults with atrial fibrillation

Thyroid Disorders and Dementia Risk: A Nationwide Population-Based Case-Control Study

BACKGROUND AND OBJECTIVES

Dementia has been gaining attention in aging societies and is estimated to affect 50 million adults globally in 2020, and 12% of the US population may develop a thyroid disorder in their lifetime. There have been limited studies investigating the correlation between thyroid disorder and dementia in the Asian population.

METHODS

Our large nationwide population-based case-control study utilized the Taiwanese National Health Insurance Research Database. 7,843 adults with newly diagnosed dementia without a previous history of dementia or neurodegenerative disease between 2006 and 2013 were identified and included in our study. 7,843 adults without dementia diagnosis prior to the index date were age and gender matched as controls. Diagnosis of hyperthyroidism or hypothyroidism prior to the diagnosis of dementia or the same index date was identified. Results were obtained from logistic regression models and adjusted for sex, age, history of hypertension, diabetes, coronary artery disease, depression, hyperlipidemia, alcohol dependence syndrome, tinnitus, hearing loss, and radioactive iodine treatment.

RESULTS

A total of 15,686 patients were included in the study. Both case and control groups were slightly predominantly female (4,066 [51.8%]). The mean (SD) age for those with dementia was 74.9 (11.3) years, and for those without dementia was 74.5 (11.3) years. Among patients aged 65 years or older, a history of hypothyroidism was associated with an increased risk of being diagnosed with dementia (aOR, 1.81; 95% Cl 1.14-2.87; p=0.011), which was an association not present in patients older than 50 years but younger than 65 years. We found that this association was most significant among patients aged 65 years or older with a history of hypothyroidism who received hypothyroidism medication (aOR, 3.17; 95% Cl 1.04-9.69; p=0.043).

DISCUSSION

Our large-scale case-control study found that among people ≥ 65 years old, those with a history of hypothyroidism were associated with an 81% increased risk of having dementia and among those, there was an over 3-fold increased dementia risk with thyroid conditions that required thyroid hormone replacement treatment. Future well-controlled prospective longitudinal studies should be conducted to elucidate these potential mechanisms and relationships.

CLASSIFICATION OF EVIDENCE

This study provides Class III evidence that among patients aged 65 years or older, a history of hypothyroidism was associated with an increased risk of being diagnosed with dementia.

Spectrum of thyroid dysfunction and dementia: a dose-response meta-analysis of 344,248 individuals from cohort studies

Thyroid hormone, as a modifiable risk factor for dementia, promotes neurocognitive function and regulates metabolic processes. Various studies have defined different thyroid-stimulating hormone cutoffs, but the safest thyroid-stimulating hormone concentration was absent. A dose-response meta-analysis describing the overall functional relation and identifying exposure intervals associated with a higher or lower disease risk is thus desirable. Therefore, our current analysis was conducted to understand the influence of thyroid dysfunction on dementia risk. We searched PubMed, Embase, and Web of Science before May 1, 2020 for human studies published in English. Studies were considered for inclusion if they used a cohort study design to measure the risk of dementia in different thyroid function status groups, diagnosed thyroid functional status and all-cause dementia, included participants aged >18 years, and provided quantitative measures of data. The analysis contained 17 articles with 344,248 individuals with a 7.8-year mean follow-up. Ten studies with 329,287 participants indicated that only subclinical hyperthyroidism was associated with an increased risk of dementia. In contrast, subclinical hypothyroidism, clinical hyperthyroidism, and clinical hypothyroidism did not affect dementia. In the dose-response meta-analysis with 46,417 samples from 11 studies, the association of thyroid-stimulating hormone with the risk of dementia exhibited a U-shaped curve. Our study indicated that subclinical hyperthyroidism was associated with the risk of dementia and the thyroid-stimulating hormone concentration at around 1.55-1.60 mU/L as the optimum range for the risk of dementia.