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 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/kg x day) and placebo tablets, 3) daily T3 administration (40 microg on even dates, 20 microg on uneven dates) plus placebo injections, and 4) daily GH injections plus T3 administration. GH administration increased both free T3 (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]. T3 administration yielded comparable levels of FT3 (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, P < 0.01). Significant increments in serum levels of insulin-like growth factor I and insulin were recorded with GH administration, but not with T3 alone. Resting heart rate increased to a similar degree after GH administration and T3 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 T4 to T3.