The effect of recombinant human growth hormone (rhGH) administration on thyroid function was studied in 14 girls with Turner's syndrome for 18 months. All patients were euthyroid and had no documented GH deficiency. Treatment with rhGH, administered as daily subcutaneous injections in a dose of 0.15 IU/kg/day, led to a decrease of serum T4 levels from 124 +/- 4 (mean +/- SEM) to 106 +/- 4 nmol/l (P < 0.05) after 6 months of therapy. Thereafter serum T4 levels returned progressively to baseline levels. Serum TBG levels increased consistently from 21.9 +/- 0.5 to 24.4 +/- 0.9 mg/l (P < 0.05) after 3 months. Serum T3 and TSH levels did not change significantly during rhGH administration. The T3/T4 ratio (x 100) increased from 1.9 +/- 0.1 to 2.2 +/- 0.1 (P < 0.05) after 6 months of treatment and returned thereafter to baseline levels. The observed changes in T3/T4 ratio demonstrated a striking parallelism with the changes in height velocity and plasma IGF-I levels. We conclude that rhGH treatment results in a transient alteration of the thyroid status in girls with Turner's syndrome. The functional importance of this phenomenon, in particular its role in the concomitant growth acceleration, remains to be established.