Calorigenic Effects on Growth Hormone: The Role of Thyroid Hormones

GH administration increases energy expenditure, independent of changes in lean body mass, in healthy, obese, and GH-deficient subjects. This may be casually link to the well known GH-induced increase in peripheral T₄ to T₃ generation, but experimental data are sparse. In this study we have addressed whether 1) the calorigenic effects of GH administration could be reproduced by oral supplementation of T₃ in a dose selected to mimic the GH-induced increase in peripheral T₃ levels; and 2) combined GH and T₃ administration have a synergistic effect on resting energy expenditure (REE).

Eight normal male subjects (aged 21-27 yr; body mass index, 21.11-27.17 kg/m² ) were randomly studied during four 10-day treatment periods with 1) daily sc placebo injections and placebo tablets, 2) daily sc GH injections (0.1 IU/kgday) and placebo tablets, 3) daily T₃ administration (40 mug on even dates, 20 mug on uneven dates) plus placebo injections, and 4) daily GH injections plus T₃ administration. GH administration increased both free T₃ (F T₃ ) levels [mean SE, 6.2 0.3 (control) vs 7.3 0.5 (GH) pmol/L; P<0.05] and REE (resting energy expenditure) [mean d SE, 1959 67 (control) vs 2164 55 (GH_ Cal/24 h; P<0.01]. T₃ administration yielded comparable levels of F T₃ (7.7 0.5 pmol/L; T₃ vs GH, P=0.37), but did not increase REE (2015 48 Cal/24 h; T₃  vs control, P=0.23). Combined GH and T₃ administration increased REE to a level higher than that seen with T₃ alone (2279 68 Cal/24 h; T₃ vs GH plus T₃ , P3 levels to mimic the expected GH-induced increment in T₃ concentrations, as observed in a previous study ^[5]. The patients were instructed to follow a weight-maintaining diet, including a minimum daily intake of 200 g carbohydrates. At the end of each period, the subjects were hospitalized and kept fasted in the supine position from 2200 h. On the following day, REE was estimated by indirect calorimetry for 30 min.

The Journal of Clinical Endocrinology and Metabolism
T. W., T. G., N.M., J. S. C, J. W., J. O. L. J  April 1, 1996, Vol. 8, No. 4