Photoperiodically generated triiodothyronin (T-3) in the mediobasal hypothalamus (MBH) has critical roles in the photoperiodic response of the gonads in Japanese quail. In a previous study, we demonstrated seasonal morphological changes in the neuro-glial interaction between gonadotrophin-releasing hormone (GnRH) nerve terminals and glial endfeet in the median eminence (ME). However, a direct relationship between photoperiodically generated T-3 and seasonal neuro-glial plasticity in the ME remained unclear. In the present study, we examined the effect of T-3 implantation into the MBH on the neuro-glial interaction in the ME. T-3 implantation caused testicular growth and reduced encasement of nerve terminals in the external zone of the ME. In contrast, no morphological changes were observed in birds given an excessive dose of T-3, which did not cause testicular growth. These results support the hypothesis that thyroid hormone regulates photoperiodic GnRH secretion via neuro-glial plasticity in the ME.