Evidence for Mild Thyroidal Impairment in Women Undergoing Endurance Training*

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Abstract
The effects of endurance training on body composition and the pituitary-thyroid axis were studied in 29 healthy, young (mean age, 28.7 yr), regularly menstruating women. Women who were initially jogging a mean of 13.5 miles/wk were selected for this study to minimize dropouts. Body composition, measured by hydrostatic weighing, and nonfasting plasma concentrations of T4 [thyroxine], T3 [triiodothyronine], r[reverse]T3, TSH, and TRH-stimulated TSH, measured by radioimmunoassay, were examined initially and after each subject''s weekly mileage had increased to 30 miles (.DELTA.30) for at least 2 consecutive wk. Two subjects had compensated primary hypothyroidism and were not included in the subsequent data analysis. At .DELTA.30, mean total body wt did not change, mean fat wt decreased (-1.02 kg; P < 0.005), and mean lean wt increased (+0.75 kg; P < 0.05). T4 and unstimulated TSH did not change. Mean (.+-. SE) T3 decreased from 107.2 .+-. 4.4 to 97.9 .+-. 3.4 ng/dl (P < 0.025), and mean rT3 decreased from 170.9 .+-. 13.9 to 154.6 .+-. 13.2 pg/ml (P < 0.025). The decreases in T3 and rT3 were accompanied by significantly greater TSH responses to TRH stimulation [mean (.+-. SE) area under TSH curve, 1381.4 .+-. 123 vs. 1712.8 .+-. 202 .mu.IU/ml .cntdot. min; P < 0.01]. Physically active women who undergo additional endurance training become more lean without a change in total body wt, have changes in T3, rT3, and TRH-stimulated TSH indicative of mild thyroidal impairment.