The effect of acute exercise session on thyroid hormone economy in rats

The hypothalamic-pituitary-thyroid axis is affected by acute exercise, but the mechanisms underlying thyroid function changes after exercise remain to be defined. The aim of this study was to elucidate the effects of a session of acute exercise on the treadmill at 75% of maximum oxygen consumption on thyroid function of rats. Male Wistar rats were divided into five groups: control (without exercise), and killed immediately after (0 min) or 30, 60, and 120 min after the end of the exercise session. A significant increase in serum tri-iodothyronine (T(3)) occurred immediately after the exercise, with a gradual decrease thereafter, so that 120 min after the end of the exercise, serum T(3) was significantly lower than that in controls. Total thyroxine (T(4)) increased progressively reaching values significantly higher than that in the control group at 120 min. T(3)/T(4) ratio was significantly decreased 60 and 120 min after the exercise, indicating impaired T(4)-to-T(3) conversion. Liver type 1 deiodinase activity (D1) significantly decreased at 60 and 120 min, while pituitary D1 increased progressively from 30 to 120 min after the exercise, and thyroid D1 was increased only immediately after the end of the exercise. Brown adipose tissue (BAT) type 2 deiodinase activity (D2) was significantly lower at 30 min, but pituitary D2 remained unchanged. No change in serum thyrotropin was detected, while serum corticosterone was significantly higher 30 min after the exercise. Our results demonstrate that decreased liver D1 and BAT D2 might be involved in the decreased T(4)-to-T(3) conversion detected after an exercise session on the treadmill.