The effect of exercise training intensity on thyroid activity at rest.

The influence of exercise training intensity on thyroid activity at rest was studied in male Wistar rats, weighting 114 g +/- 24 (mean +/- SD) at the beginning of the experiment. Animals were assigned to the following groups: untrained controls and rats trained on a treadmill at the speed of 20 m/min over a 5-week period with different intensities: 2 x 60 min weekly, 4 x 60 min, 6 x 20 min, 6 x 40 min and 6 x 60 min weekly. Thyroid peroxidase (TPO) and hepatic iodothyronine 5'-monodeiodinase (5'DI) activities as well as plasma thyroxine (T4), 3,3'5-triiodothyronine (T3) and 3,3'.5'-triiodothyronine (rT3) concentrations were determined. Training intensity was found to influence parameters under investigation. TPO activity was decreased in groups trained 240 min (4 x 60 min and 6 x 40 min) and 360 min (6 x 60 min) weekly in comparison to control, untrained group. Furthermore, a drop in T4 plasma concentration in all trained groups and a decrease in T3 plasma concentration in groups exercising for 120 min (2 x 60 min and 6 x 20 min) weekly, as compared to control, untrained rats, was found. Hepatic 5'DI activity and rT3 plasma concentration were not affected by training. Thus, exercise training in rats seems to elicit the fall in TPO activity and T4 plasma concentration at rest but without changing hepatic 5'DI activity and rT3 plasma concentrations. A decline in T3 plasma concentration, observed in rats trained with the lowest exercise intensities, cold be regarded as transitional effect in adaptation to chronic exercise.