Thyroid Axis Adaptations to Moderate Short-term Energy Restriction in Healthy, Young Women

Abstract Objective Short-term dieting has gained popularity in women. We studied thyroid hormone change after short-term, moderate energy restriction. Methods Nineteen, healthy women aged 23.36 ± 2.08 yr (mean ± SD) without obesity and thyroidal disease underwent a neutral (NEA, ±0%) and a deficient (DEA, -55%) diet for five days each in the early follicular phases of successive menstrual cycles. Blood was sampled every 10 minutes between 8 AM and 4 PM and analyzed for TSH, GH, and cortisol every 30 min, total T3 (TT3), reverse T3 (rT3), and total T4 (TT4) hourly and free T3 (fT3), free T4 (fT4) and TBG at the beginning and end of the studies. Liquid chromatography-tandem mass spectrometry (LCMS) assessed all thyroid hormones except TSH and TBG. Data shown as mean difference or least squared (ls) mean (±sem). Results There was a small decrease in BMI and body weight after DEA (0.4±0.08 kg/m2; p<0.001 and 1.1 ± 0.21 kg; p<0.001, respectively), with unchanged fat mass. Compared to NEA, TT3 (ls mean ± SEM; 95.55 ± 2.89 ng/dL vs. 89.15 ± 2.89; p<0.0001), and TSH (1.03 ± 0.07 vs. 0.92 ± 0.07 μIU/mL; p<0.0001) declined while TT4 (6.06 ± 0.25 vs. 6.26 ± 0.25 μg/dL; p=0.005), fT4 (1.71 ± 0.07 vs. 1.83± 0.07 ng/dL; p=0.0052) and rT3 (9.02 ± 0.56 vs. 12.04 ± 0.56 ng/dL; p<0.0001) increased after DEA with no change in TBG, GH and cortisol. Conclusion Adaptive central and peripheral changes in thyroid hormones occur after short-term, moderate dieting in young women without obesity. ClinicalTrials.gov NCT02858336