It is clinically evident that women suffering from thyroid disorders are associated with frequent occurrence of menstrual disturbances and impaired fertility, and these abnormalities are improved by restoring the euthyroid state. The exact mechanism for such reproductive aberrations is not well known; however, it is conceivable that thyroid hormones might have a direct role in ovarian physiology via receptors in granulosa cells. We evaluated the effect of thyroid hormones (T3,T4) on the proliferative activity, apoptosis and function of the human granulosa cells COV434. The cells have been treated with T3 and T4 (10−8M) after dose response analysis. Cell viability, number and apoptosis of treated cells were evaluated by MTT assay, by cell counting, and by TUNEL assay. T3 and T4 were able to induce cell growth and viability. In particular T3 showed the strongest effect, being able to induce a 40% increase in the cell number after 72 h of treatment. To better define the observed effects, cell cycle profile by FACS analyses has been performed. When the cells were cultured in serum-free condition to induce apoptosis, T3 was able to induce a decrease in the cell apoptotic rate of 30% and the PI3K pathway seems to be involved in the survival effect of T3. The ultrastructure of the cells exposed to THs has also been analysed by electron microscopy. In addition the treated cells showed a strong increase in the relative production of 17 beta estradiol, which was increased of 50% by the T3, and of 30% by T4 treatments. These results support our hypothesis that THs influence cell proliferation in human ovary granulosa cells.