In this study of 409 euthyroid patients with AD, there were significant associations between T4 and functional ability. Higher T4 levels were found in those individuals who were more dependent with activities of daily living (including personal care), but no such association was seen with TSH. There was a nonsignificant trend to worse mood factor scores as identified by the NPI, in patients with higher T4 concentration. No significant relationships were identified between cognitive functioning and frontal or psychotic symptoms with thyroid hormones.
Our study is exploratory with several limitations. First, measures more sensitive to anxiety and depression may have demonstrated an effect on mood. Secondly, triiodothyronine (T3) levels may be more closely associated with cognitive dysfunction in AD, though a considerable proportion of T3 is metabolised from T4 within the brain (Bunevicius, 2009). Thirdly, this cross sectional study also does not permit examination of any association between thyroid hormone concentrations at the time of diagnosis of AD, and cognitive changes over time. Future studies might address these limitations.