INDUCTION OF LOW-T-3 SYNDROME IN EXERCISING WOMEN OCCURS AT A THRESHOLD OF ENERGY AVAILABILITY

To investigate the relationship between energy availability (dietary e nergy intake minus energy expended during exercise) and thyroid metabo lism, we studied 27 untrained, regularly menstruating women who perfor med similar to 30 kcal.kg lean body mass (LBM)(-1).day(-1) of supervis ed ergometer exercise at 70% of aerobic capacity for 4 days in the ear ly follicular phase. A clinical dietary product was used to set energy availability in four groups (10.8, 19.0, 25.0, 40.4 kcal.kg LBM(-1).d ay(-1)). For 9 days beginning 3 days before treatments, blood was samp led once daily at 8 A.M. Initially, thyroxine (T-4) and free T-4 (fT(4 )), 3,5,3'-triiodothyronine (T-3) and free T-3 (fT(3)), and reverse T- 3 (rT(3)) were in the normal range for all subjects. Repeated-measures one-way analysis of variance followed by one-sided, two-sample post h oc Fischer's least significant difference tests of changes by treatmen t day 4 revealed that reductions in Tg (16%, P < 0.00001) and fT(3) (9 %, P < 0.01) occurred abruptly between 19.0 and 25.0 kcal.kg LBM(-1).d ay(-1) and that increases in fT(4) (11%, P < 0.05) and rT(3) (22%, P < 0.01) occurred abruptly between 10.8 and 19.0 kcal.kg LBM(-1).day(-1) Changes in T-4 could not be distinguished. If energy deficiency suppr esses reproductive as well as thyroid function, athletic amenorrhea mi ght be prevented or reversed by increasing energy availability through dietary reform to 25 kcal.kg LBM(-1).day(-1), without moderating the exercise regimen.