Introduction & Objective: The normal females reproductive function , needs hypothalamus-hypophysis-ovarian extensive hormonal messages. Primary hypothyroidism is characterized by reduced production and secretion of thyroid hormones. During follicular growth PCNA (Proliferating Cell Nuclear Antigen) and cycklin D complex play an important role in regulating cell proliferation .This study aimed to determine the cell proliferation index and how this process changes induced by thyroid hormone decreased in rat ovarian follicles.

Materials & Methods: In this experimental study, 20 Wistar female rats were divided into experimental and control groups. Experimental group was chemically thyroidectomized by administering propylthiouracil (PTU) (500 mg per liter of drinking water). The control group received normal drinking water. After three weeks rats were killed and their ovaries dissected and fixed for the histological preparation. Cell proliferation was determined by PCNA and stereological methods were used for counting cells.

Results: Cell proliferation index showed a significant decrease in the frequency of follicular growth from prenatal to graafian follicles in hypothyroidism groups(P<0.05). However, a slight increase was observed in follicles with extensive antrum but it was not significant(P>0.05) . PCNA expression determined that Primary follicle growth begins earlier. Positive PCNA cells were not observed in primordial follicles of the groups.

Conclusion: According to the results of our study, this hypothesis is raised that granulosa cells in growing follicles may be increased by follicle adjacent cells in ovarian stroma . Hormonal changes following the reduction of thyroid hormones may greatly affect the cell proliferation index and lead to faster follicle degeneration.