Effects of growth hormone replacement therapy on levels of cortisol and cortisol-binding globulin in hypopituitary adults

Objective: To determine if human growth hormone (hGH) replacement therapy alters pharmaco-kinetics of hydrocortisone (CS) substitution in hypopituitary adults. Design: To this aim, we analysed serum and salivary CS profiles 270 min after oral CS administration at baseline and 6 and 12 months after initiation of hGH replacement therapy. Methods: Serum IGF-I, cortisol-binding globulin (CBG), thyroxine-binding globulin (TBG) and sex hormone-binding hormone (SHBG) were measured using commercially available radioimmunoassays. In-house immunofluorometric assays were employed for measurements of CS and hGH. Results: hGH replacement did not change total serum CS bioavailability (area under the serum cortisol profile curve). Interference of orally administered CS with salivary measurement of free CS (fCS) caused significant bias. Therefore, fCS levels were calculated from their total CS and cortisol-binding globulin (CBG) levels. CBG decreased by approximately 30% after both 6 and 12 months of hGH replacement therapy (n = 20, P < 0.01). A significant negative correlation between ΔCBG (CBG_6months - CBG_baseline) and ΔIGF - I (IGF - I_6months - IGF - I_baseline) was observed (P = 0.04). The calculated values of free CS tended to increase with physiological hGH replacement, but this effect was marginal and did not reach statistical significance. In contrast to the CBG concentrations, plasma levels of sex hormone-binding globulin and thyroxine-binding globulin were essentially stable. Conclusion: Given that no clinically relevant alterations in pharmacokinetics of CS were evoked by initiation of hGH replacement in hypopituitary adults, we conclude that CS substitution does not require dose adjustment after initiation of hGH replacement.