We examined the effects of T3 and T4 on basal and FSH-induced aromatase activity in granulosa cells isolated from medium-sized follicles (4-6 mm) of prepubertal pigs. Treatment of cells with T3 or T4 alone during an initial 48-h induction period did not result in any significant change in aromatase activity, as measured by 17 beta-estradiol accumulation during the subsequent 6-h test period, when testosterone (0.5 microM) was added as substrate. However, when cells were cultured with FSH and T3 or T4 during the induction period, a definite dose-dependent inhibition of FSH-induced aromatase activity was demonstrated. This inhibition was not altered by the presence of a binding protein (BSA). The inhibition of FSH-induced aromatase activity by T4 and T3 is a true biological effect, as inactive iodocompounds (MIT and DIT) and iodide had no significant effect on the gonadotropin-induced aromatase activity. Furthermore, the viability of cells was unaffected by the thyroid hormones, and total cellular protein did not change significantly. These results indicate that thyroid hormones might play an important role in modulating FSH-induced aromatase activity, and that the elevated plasma estrogen levels in some cases of hyperthyroidism are not due to increased ovarian secretion.
