There are many factors involved in the aetiology of hirsutism. It is well known that some drugs may cause hirsutism as a side effect, such as phenytoin, diazoxide, minoxidil etc. In Turkey, where the endemic goiter constitutes an important health problem, the value of suppression therapy with thyroid hormone still carries a special importance. We established the increasing tendency of hirsutism in patients treated with L-thyroxine (L-T4) for various thyroid pathology. That is why we decided to evaluate the role of thyroid hormones in the aetiology of hirsutism observed in patients treated with L-T4. We determined the total and free T3, T4, TSH, TBG, Plasma Cortisol transcortin, delta-4-androstenedione, FSH, LH, prolactin, total and free testosterone, estradiol, progesterone, sex hormone-binding globulin (SHBG), dehydroepiandrosterone (DHEA), dehydroepiandrosterone sulphate (DHEAS) and 17 hydroxyprogesterone (17 OH Pg) levels in serum or plasma in a group of female patients who were taking L-T4 daily, regularly at least for 6 months and complaining of the occurrence of hirsutism. We demonstrated in these patients SHBG, transcortin and estradiol levels significantly lower than controls (p less than 0.001, p less than 0.05 and p less than 0.05, respectively) and DHEAS level significantly higher than controls (p less than 0.001). An important correlation was found between TBG and SHBG levels (r:0.536, p less than 0.05) and also between total and free testosterone levels (r:0.952, p less than 0.001).