Alterations of cognitive functions induced by exogenous application of thyroid hormones in healthy men: A double-blind cross-over study using event-related brain potentials

The influence of experimentally induced subclinical hyperthyroidism on cognitive functions was tested using exogenous thyroxine or placebo. In a double-blind cross-over design, 24 healthy young men received either 300 μg of levothyroxine or placebo for two consecutive 3-week periods. Determination of thyrotropin (TSH), total triiodothyronine (T₃), thyroxine (T₄), and sex hormone binding globulin (SHBG) confirmed the induction of subclinical hyperthyroidism. Three weeks after treatment with placebo or T₄, subjects were tested in two visual search tasks, one requiring the serial scanning of a stimulus array of eight items in order to detect a missing feature (serial task), while the other task allowed the parallel processing of eight items in order to detect an extra feature. Reaction times varied greatly between the tasks while no effect of T₄ medication was revealed on overt search behavior. The late cognitive components of the event-related brain potentials (ERP) were registered during the tasks using a multichannel recording. The late positive component (P300) to target stimuli showed a significant reduction in amplitude in the parallel search task in hyperthyroidism, while for the serial search task a significant frontal negativity was revealed for target stimuli, both suggesting an increased effort to be exerted for the execution of visual Search tasks. Thus, short periods of subclinical hyperthyroidism in the range commonly induced in the treatment of thyroid cancer patients induce distinct alterations in brain-electric activity.