OBJECTIVE There are numerous, often contradictory reports on the effects of growth hormone (GH) therapy on thyroid function. The aim of this study was to assess the effect of such therapy on serum concentrations of thyroid hormones in GH-deficient children euthyroid prior to the treatment, and to determine the necessity of thyroid hormone administration in these patients. METHODS The study included 32 GH-deficient patients in the first stage of sexual development, in whom disorders of thyroid function could be excluded. The inclusion criteria were based on clinical examination and levels of thyroxine (T4), triiodothyronine (T3), free thyroxine (fT4), free triiodothyronine (fT3), reverse triiodothyronine (rT3), thyrotropin (TSH) before and after stimulation with thyrotropin-releasing hormone (TRH). Recombinant growth hormone (rGH) (Genotropin 16U, Pharmacia) was administered at a dose of 0.7 U/kg/week. Fasting blood samples were drawn before treatment and after 3, 6, 9 and 12 months of therapy. Thyroid hormones were measured using RIA and IRMA methods. RESULTS There were no physical signs of hypothyroidism in the patients examined during 12 months of rGH administration, and the satisfactory growth rate was achieved. T4 levels decreased in the first 3 months but remained within the normal range, and then returned to the values prior to the treatment. A similar trend was observed for fF4, with 28.5% of patients exhibiting fF4 levels below the normal in the 3rd month. An increase during the first 3 months of therapy was observed in the cases of T3 (statistically non-significant) and fT3, and these values then fell to levels within the normal range of patients' age. During treatment, TSH levels decreased but remained within the normal range. CONCLUSIONS A transient decrease in T4 concentrations in the 3rd month with unchanged T3 and an increase in fT3 concentrations probably result from the effect of rGH on the peripheral metabolism of thyroid hormones. The results obtained do not support the use of thyroid hormone therapy with levothyroxine during the first year of rGH therapy in patients who are initially euthyroid.