effects of levothyroxine administration on ovulation rate and sex hormone levels in prepubertal and adult rats

thyroid gland dysfunction is associated with disorders of female reproductive functions. the aim of this study was to examine the effects of hyperthyroidism on ovulation rate and peripheral sex steroid levels in prepubertal and adult rats. two groups of female rats aging 30 days (prepubertal rats) and 60 days (adult rats) were made hyperthyroid by oral administration of levothyroxine daily. in the two control groups (n=lo) rats with the same ages received the same volume of normal saline. after 10 days the serum levels of t3, t4, lh, fsh, estradiol and progesterone were measured by ria technique and also sections of ovaries were prepared for histological studies. all ovarian follicles and corpora lutea were counted to estimate ovulation rate. the results indicated that serum estradiol and progesterone levels of adult hyperthyroid rats were significantly lower than those of the control group (p<0.05). however, there was no significant difference between estradiol levels in the prepubertal hyperthyroid group and its control group. in the hyperthyroid groups the mean number of primary, secondary and antral follicles and corpora lutea was significantly less than those of control groups. it was concluded that high levels of plasma thyroid hormones resulted in lowered body weight and disturbed ovarian follicle development and differentiation leading to reduction in ovarian steroidogenesis and ovulation.