The purpose of the study was to examine the effects of weight loss on resting metabolic rate (RMR) and on serum T3 levels in obese children and to investigate whether RMR changes are related to T3 changes. Sixty-four healthy, overweight, children (age: 12.1+/-1.1 years, body mass index 29.3+/-4.3 kg/m2) were studied during a 6-week weight reduction programme. RMR (by indirect calorimetry) total T3, total T4, TSH and fat-free mass (FFM) (by anthropometry) were measured at baseline and after 6 weeks of dietary treatment. Weight loss resulted in a 10.1% decline in RMR (P < 0.01) and a 23.4% decrease in serum T3 levels (P < 0.001). RMR was correlated with FFM before (r = 0.78, P < 0.001) and after weight loss (r = 0.76, P < 0.001). The changes in RMR were positively correlated with the changes in FFM (r = 0.48, P < 0.05) but also with the changes in serum T3 levels (r = 0.47, P < 0.05). The initial T3 levels predicted the subsequent fall in T3 that occurred after 6 weeks of dietary treatment (r = -0.60, P < 0.001).
Conclusions: A significant decrease in serum T3 concentrations and resting metabolic rate occurred as a result of a 6-week weight reduction programme in an obese child population. The decline in T3 levels combined with fat-free mass loss could be responsible for the reduction in resting metabolic rate.