Hyperthyroidism is associated with decreased libido, impotence and gynaecomastia due to raised levels of oestradiol suggesting a hyperestrogenemic environment. In adult rodents sexual dimorphism of hypothalamo-pituitary axis is complete and cannot be influenced by any epigenetic influence. Therefore, under no environmental and epigenetic influence can the sexual dimorphism of the pituitary gland be abolished in these animals. To find out whether such sexual dimorphism of hypothalamo-pituitary axis exists in humans too, we studied the relationship of the peripheral gonadal steroid with the circulating gonadotropins, markers of hypothalamo-pituitary axis function in hyperestrogenic, hyperthyroid males. We chose a disease model, as experimental hyperestrogenism cannot otherwise be induced in healthy human male subjects