To study the action of human growth hormone (hGH) on peripheral metabolism of serum thyroxine (T4), an oral loading dose of levothyroxine (1.2 mg/m2) was administered to seven children with hypopituitarism before initiation of hGH therapy. Serum concentrations of triiodothyronine (T3), T4, reverse triiodothyronine (rT3), and thyroxine-binding globulin (TBG) capacity were measured sequentially for 6 days. The study was repeated after 4 wk of treatment with hGH. Serum concentrations of T4were not affected by hGH therapy. In contrast, mean basal serum concentration of T3increased significantly after treatment with hGH. Also, changes in serum concentrations of T, and in the ratio of T3/T4 after an oral dose of levothyroxine were significantly augmented during hGH therapy. Serum concentrations of rT3changed in the opposite direction of T3during therapy. After treatment with hGH, the mean basal level of serum rT3, decreased, and increases in serum concentrations of rT3, after oral levothyroxine were significantly attenuated. No changes in mean serum concentrations of thyroid stimulating hormone (TSH) and TBG capacity were observed.