Some of the Authors previously demonstrated a significant precocious serum T3 increase after 200 micrograms TRH acute intravenous administration (TRH test). Reverse-T3 (rT3) is now known to interfere with T4 conversion to T3. We therefore compared spontaneously occurring to TRH test-induced changes in T3 and rT3 serum levels within a group of four healthy women in fertile age. Maximum rT3 increase during TRH test did not differ significantly from the maximum spontaneous variation at the same time of the day. Maximum T3 increase, on the contrary, was significantly higher than observed maximum spontaneous variation (0,81 ng/ml versus 0,39 ng/ml increase, p less than 0,01). Possible implications are discussed in the text.