Independent and combined associations of diabetes and thyroid disorders with risks of incident dementia

Guo J, Li J, Bin Gao, Ke Liu, Xiaohui Sun, Y P Ding, Yingying Mao

Background Diabetes and thyroid disorders are modifiable risk factors for cognitive impairment. However, no studies have been conducted to comprehensively examine associations of different types of the two diseases, as well as their combinations, with dementia risks. Methods Based on the data from UK Biobank study, patients with different types of diabetes and thyroid disorders were identified with the hospital inpatient record. Status of dementia was assessed with multi-source information. Independent and combined associations of diabetes and thyroid disorders with dementia were estimated with Cox regression models. Results Among 284,081 participants, 8,236 incident cases of all-cause dementia (ACD) [3,755 Alzheimer’s disease (AD) cases, 1,893 vascular dementia (VD) cases] were identified over a mean [standard deviation (SD)] follow-up of 13.03 (2.29) years. Four types of diabetes and two types of thyroid disorders were significantly associated with increased risks of dementia. Compared with participants without diabetes and thyroid disorders, those with only thyroid disorders [hazard ratio (HR) = 1.33, 95% confidence interval (CI) = 1.21 to 1.47, P < 0.001], only diabetes (HR = 1.76, 95% CI = 1.64 to 1.89, P < 0.001), and both (HR = 2.16, 95% CI = 1.84 to 2.53, P < 0.001) had elevated risks of ACD ( P trend < 0.001), as well as higher risks of AD and VD. Combined associations of hypothyroid and diabetes and of hyperthyroid and diabetes were also significantly positive (all P values < 0.05). Moreover, cumulative years of diabetes and thyroid were related to higher risks of dementia. Conclusions Diabetes, thyroid disorders, and their combinations were associated with increased risks of dementia in adults aged 55 years and older.