We examined the effect of growth hormone (GH) therapy on thyroid function in 57 children with isolated GH deficiency and whether this effect could influence their growth response. Thyroid function and insulin-like growth factor I levels were measured before and after 3, 6, and 12 months of recombinant-GH therapy (20 U/m2 per week, given subcutaneously), after a 1-month withdrawal from therapy, and after a further 6 months of GH administration. The serum concentration of triiodothyronine (T3) and the T3/T4 (thyroxine) ratio increased after 12 months of GH treatment, whereas total T4 and free T4 levels decreased; thyrotropin levels did not change significantly during treatment but increased after a 1-month withdrawal. After a further 6 months of GH therapy, an increase in T3 levels and in the T3/T4 ratio and a decrease in total T4 and free T4 levels were found again, and thyrotropin levels decreased. The increment in growth velocity after 12 months of therapy correlated positively with the T3/T4 ratio and negatively with total T4 and free T4 values. These data confirm in children a GH-induced enhancement of peripheral conversion of T4 to T3. This effect appears to be more evident in children who are most sensitive to GH in terms of growth-promoting activity.