ObjectivesTo investigate the effects of 12-week high-intensity- (HIIT) vs. moderate-intensity-interval training (MIIT) on thyroid stimulating hormone (TSH) and thyroxine (T4) and insulin-resistance in overweight/obese adolescent girls.MethodsTwenty 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.ResultsFollowing 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).ConclusionTwelve 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.

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