Thyroid Function in Normals: Influences on the Electroencephalogram and Cognitive Performance

Recently it has been suggested that thyroid function may influence neuropsychological processes not only as a result of thyroid disease but also through normal neurophysiologic adaptive mechanisms. In 69 normal university students, levels of thyroid stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3) were assayed in blood samples and related to measures of cognitive performance and to spectral measures of the EEG recorded during cognitive performance. Significant relationships were found between T3 levels and delta power in the EEG from occipital leads, with higher T3 associated with less EEG power in the left hemisphere and greater power in the right hemisphere. Higher T3 levels were associated with poorer performance on several cognitive tasks, particularly for males, although the males with higher T3 levels also showed faster correct responses on a word fluency task. These results seem to support the hypothesis that an individual's characteristic level of thyroid function is relevant to normal neuropsychological processes.