Converging evidence suggests a possible link between thyroid state and Alzheimers disease (AD), including a higher probability of dementia in individuals with higher TSH levels and a two-fold risk of AD in patients with hypothyroidism. Thyroid hormones modulate factors associated with AD, including amyloid precursor protein expression in the brain, suggesting a possible role for thyroid hormone in AD pathology. The present study is the first to directly evaluate brain thyroid hormone levels in AD. Triiodothyronine (T ) and thyroxine (T ) levels were measured with radioimmunoassay (RIA) in post-mortem samples of prefrontal cortex of patients with pathologically confirmed AD, including Braak stage I-II (n=8), Braak stage V-VI (n=8), and controls without any primary neurological disease (n=8). T levels did not differ between groups. T levels were significantly lower in Braak stage V-VI brains relative to controls, but there was no statistically significant difference between T levels in Braak stage I-II versus controls. Results suggest that the conversion of T to T may be affected in advanced AD, perhaps due to alterations in deiodinase activity. Reduced conversion of T to T3 in AD may be associated with both AD pathology and the clinical presentation of dementia.