Background: Optimal body composition and body weight (BW) are important performance factors in sports that emphasize aesthetics and high power to weight ratio. Weight reduction guidelines for athletes per date suggest a weekly weight loss of 0.5-1.0 kg which corresponds to 0.7% and 1.4% of BW in a 70-kg athlete. Weight reduction may lead to loss of lean body mass (LBM) and changed thyroid hormone (TH) profile and thereby reduced basal metabolic rate (BMR). Combining energy restriction with resistance training may inhibit loss of LBM in obese subjects. Little is known about these changes and their effect on metabolism (BMR, TH) in athletes. Purpose: To compare changes in body composition, TH and BMR in two different weight-loss interventions promoting loss of 0.7% versus 1.4% of BW per week in elite athletes. Methods: 30 male and female elite athletes were randomized into slow weight reduction [ SLOW , n=14, 23.5 (3.3) y, 72.1 (12.2) kg] and fast weight reduction [ FAST , n=16, 22.3 (4.7) y, 72.2 (11.3) kg]. Both intervention groups included energy restricted diets and resistance training regimen for 4-12 weeks. The duration of the intervention was dependent on the desired weight loss and the type of intervention group the participant was allocated to. Diets were recorded at baseline by 4-day weighed food records which were used to design individual intervention diets promoting weekly BW loss of 0.7% or 1.4%. Four weekly resistance training sessions were added to the usual training regimen to stimulate muscle hypertrophy. The following measurements were taken at baseline and post intervention: BW, LBM, fat mass (FM) with dual energy x-ray absorptiometry (DEXA), serum thyroid stimulating hormone(s-THS) and free thyroxine (s-free T4) and BMR with indirect calorimeter. Results: There were no significant differences between the groups in any variables at the baseline. BW was reduced by 5.6 (3.0) % [mean(SD)], p<0.001 and 5.4 (2.3%), p<0.001) for SLOW- and FAST weight reduction group, respectively. The changes were not significantly different between the intervention groups (p=0.8). LBM was significantly increased for SLOW [2.0 (1.3) %, p<0.001] but not for FAST [1.1 (3.0) %, p=0.3]. There was a significant difference in FM reduction obtained during weight loss between the intervention groups SLOW -31 (2.9) %, p<0.001) and FAST -23.4 (13.8) %, p<0.001 (p=0.04). Free T4 reduction during the intervention was significant in FAST [-11.2 (7.6) %, p=0.001] but not in SLOW. These changes were significantly different between the groups (p<0.001). BMR was reduced significantly for both SLOW [12.3 (11.4) %, p=0.036] and FAST [9.8 (6.5) %, p=0.018], but the changes did not differ between the intervention groups (p=0.9). Changes in LBM were associated with baseline LBM (r=0.49, p=0.006) but not with weight reduction rate. Changes in free T4 or LBM were not associated with changes in BMR. Conclusion: Body composition changes were different between the groups; LBM increased only in the SLOW group who also obtained a larger FM reduction than the FAST group, thus body composition changes were more beneficial in SLOW weight loss intervention. Only FAST weight reduction had an impact on thyroid hormones, producing a larger decline in free-T4 but not in TSH compared with SLOW intervention. BMR reductions did not differ between the interventions. Thus for optimal body composition changes in athletes a moderate energy restriction combined with resistance training leading to a weekly weight loss of 0.7% seems preferable. Despite maintained or increased LBM, and maintained free T4 athletes experienced reductions in BMR.