Recombinant human GH replacement therapy and thyroid function in a large group of adult GH-deficient patients: when does L-T(4) therapy become mandatory?

The effect on thyroid function of GH administration to 66 adult patients with severe GH deficiency was studied. Seventeen patients were euthyroid, and 49 had central hypothyroidism and were adequately treated with l-T4. Forty patients were assigned to a low recombinant human GH (rhGH) regimen (3 μg/kg body wt·d for 3 months followed by 6 μg/kg body wt·d for another 3 months) and 26 to a higher one (6 μg/kg body wt·d for 3 months followed by 12 μg/kg body wt·d for another 3 months). Serum IGF-I, TSH, free T4 (FT4), free T3 (FT3), reverse T3, T4-binding globulin, and antithyroid autoantibody (TgAb and TPOAb) were measured in basal condition and after 3 and 6 months of therapy. Normalization of IGF-I levels was obtained after 6-month rhGH treatment in 67% of patients, independently from the dose, whereas a significant reduction in FT4 and reverse T3 levels was recorded (P < 0.01), without variations in all the other parameters studied, including serum TSH, FT3, and T4-binding globulin circulating levels. Antithyroid autoantibodies were detected in 11 of 66 patients (16.6%). Eight of 17 (47%) euthyroid subjects and 9 of 49 (18.3%) central hypothyroid patients, despite adequate substitution at baseline, showed FT4 levels under the normal range at the end of the study. Altogether, 17 of 66 patients (25.7%) worsened their thyroid function. This study shows that GH deficiency masks in a consistent number of adult patients a state of central hypothyroidism. Therefore, during rhGH treatment, a careful monitoring of thyroid function is mandatory to start or adjust l-T4 substitutive therapy.