Effects of ovariectomy (ovx) and ovx with estradiol benzoate (EB) therapy on total and free T3 (FT3), T4, and [125I]T3 disappearance rate; basal and post-TRH, TSH, T3, and T4 concentrations were studied in female rats. Two weeks after ovx, no significant change in hormone concentrations was observed. Two micrograms of EB daily for 1 week significantly increased basal serum TSH by 40% and serum total T3 by 25–30%, but decreased serum T4 by 50%. The percentage of free FT3 and free T4 (FT4) did not differ in the shamoperated intact, ovx, and ovx-EB-treated animals, resulting in an increase in total absolute FT3 and a decrease in FT4 by EB. [125I]T3 plasma disappearance rate was not altered by either ovx or EB. Ten minutes after TRH, serum TSH increased 6- to 7-fold in both the intact and ovx rats, but increased 10- to 13-fold in the EB-treated animals. Two hours after TRH, a significant increase in serum T3 concentration was observed in all groups of animals, but the absolute incremental T3 response after EB...