Calorigenic effects of 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 causally linked to the well known GH-induced increase in peripheral T-4 to T-3 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 T-3 in a dose selected to mimic the GH-induced increase in peripheral T-3 levels; and 2) combined GH and T-3 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/m(2)) 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/kg . day) and placebo tablets, 3) daily T, administration (40 mu g on even dates, 20 mu g on uneven dates) plus placebo injections, and 4) daily GH injections plus T-3 administration. GH administration increased both free T-3 (FT3) levels [mean +/- SE, 6.2 +/- 0.3 (control) vs. 7.3 +/- 0.5 (GH) pmol/L; P < 0.05] and REE [mean +/- SE, 1959 +/- 67 (control) vs. 2164 +/- 55 (GH) Cal/24 h; P < 0.01]. T-3 administration yielded comparable levels of FT3 (7.7 +/- 0.5 pmol/L; T-3 vs. GH, P = 0.37), but did not increase REE (2015 +/- 48 Cal/24 h; T-3 vs. control, P = 0.23). Combined GH and T-3 administration increased REE to a level higher than that seen with T-3 alone (2279 +/- 68 Cal/24 h; T-3 vs. GH plus T-3, P < 0.01). Significant increments in serum levels of insulin-like growth factor I and insulin were recorded with GH administration, but not with T-3 alone. Resting heart rate increased to a similar degree after GH administration and T-3 supplementation, respectively. Tympanic temperature remained unaltered in all four studies. The results suggest that the calorigenic effect of GH is not mediated solely through increased conversion of T-4 to (3).