Thyroid gland hormones influence thymic structure. The outflow of T cells the thymus is reduced in thyroid hypofunction. In this study, experimental hypothyroidism was induced in 30, 45 and 60 days old female rats by intraperitoneally injections of propilthiouracil (10mg/kg day). In the light and electron microscopic investigation, the different thymic influence were observed among age groups. The thymic weight of experimental groups were lower than controls. The apoptotic lymphocytes, macrophages which phagocytosed apoptotic bodies, plasma cell and eosinofil around blood vessel, dilated vessels in the gland and septum, mast cells which increased as number in septum were determined in the thymic structure. Catalase activity and G-6-PD activity in plasma were decreased while malonaldehyde (MDH) activities in plasma and tissue was increased. The drugs which lead to hypothyroidism caused involution in thymus, this involution was occured extremely in adult group.