Cognitive disturbance is commonly associated with disorders of the thyroid gland, particularly hypothyroidism, and usually subsides following thyroid hormone replacement therapy. In contrast, the effects of thyroid hormones on cognitive functions in healthy individuals have rarely been studied. The goal of this open-label study was to investigate the short-term effects (duration of administration 45 days on average) of supraphysiological doses of L-thyroxine (L-T(4)) on cognitive performance in young, euthyroid, healthy subjects. Eleven subjects performed a comprehensive neuropsychological test battery, once without and once during administration of supraphysiological doses of L-T(4). There were no significant differences in any of the cognitive test results between the two test sessions. The results of this study do not support our working hypothesis that thyroid hormone can change cognitive performance in young, euthyroid, healthy individuals.