In order to determine whether elevations in serum 3,3'-diiodothyronine (3,3'T2) concentrations influence the hypothalamic-pituitary--thyroid axis, thyrotropin (TSH) and prolactin responses to thyrotropin-releasing hormone (TRH) were assessed in five patients both prior to and during 3,3'T2 administration. Mean (+/- SE) peak TSH responses to TRH were 168 +/- 64 microU/ml during 3,3'T2 administration and 168 +/- 65 muU/ml during 3,3'T2 administration. Mean basal and peak prolactin concentrations after TRH were 6 +/- 3 ng/ml and 54 +/- 26 ng/ml, whereas during 3,3'T2 administration the basal and peak prolactin levels were 6 +/- 2 ng/ml and 55 +/- 28 ng/ml, respectively. Hypothyroid rats administered triiodothyronine (10 migrogram b.i.d.) for 5 days had a mean TSH response to TRH stimulation of 0.051 +/- 0.003 mU/ml, whereas rats to whom saline or 3,3'T2 (50 microgram b.i.d.) had been given for the same time interval had mean TRH-induced TSH responses of 1.127 +/- 0.179 mU/ml and 1.324 +/- 0.286 mU/ml, respectively. None of the TSH or prolactin responses to TRH, in either human or rat studies, were apparently altered by 3,3'T2. These observations suggest that elevation of serum 3,3'T2 levels are not associated with alterations in the hypothalamic--pituitary--thyroid axis in the experimental systems employed.