Greater effects of high- compared with moderate-intensity interval training on thyroid hormones in overweight/obese adolescent girls

Objectives: To investigate the effects of 12-week highintensity- (HIIT) vs. moderate-intensity-interval training (MITT) on thyroid stimulating hormone (TSH) and thyroxine (T4) and insulin-resistance in overweight/obese adolescent girls. Methods: Twenty four adolescent girls (age 16.5 +/- 1.36 yrs) were randomly allocated into three groups: (1) HIIT (2 blocks per session of 6-8 bouts of 30 s runs at 100-110% maximal aerobic speed (MAS), with 30 s active recovery between bouts at 50% MAS; n=8), (2) MIIT (2 blocks per session of 6-8 bouts of 30 s runs at 70-80% MAS, with 30 s active recovery between bouts at 50% MAS; n=8) and (3) control group (no exercise, n=8). Each training groups engaged in three sessions per week during three months. Anthropometric parameters, aerobic capacity, homeostasis model assessment index for insulin resistance (HOMA-IR) as well as plasma TSH and T4 levels were assessed in all subjects before- and after-training. Results: Following both training programs, body mass, body mass index Z-score, waist circumference and body fat decreased, while aerobic capacity increased. However, TSH and T4 concentrations decreased only after the HIIT (-30.47%, p<0.05, ES=1.42 and -12.86%, p<0.05, ES=1.18; respectively). The HOMA-IR decreased in both training groups (-26.25%, p<0.05, ES=1.87 for MIIT and -21.72%, p<0.05, ES=2.14 for HIIT). Conclusion: Twelve weeks of HIIT was effective in reducing circulating TSH and T4 levels, unlike MIIT, in overweight/obese adolescent girls. These findings indicated that the stimulation of pituitary-thyroid function is more sensitive to training intensity than training duration. Further studies are needed to confirm this conclusion.