Background:Thyroid hormones are key substances in normal homeostasis, having variable influence on cell metabolism on different organs. Exercise is a stressful situation that
challenges this homeostasis, and one of the systems affected is the hypothalamic-pituitary-thyroid axis. While data have been reported on effects of exercise on thyroid hormones metabolism, the
results have been inconsistent or even contradictory. These divergent results may be due to differences on exercise (intensity, duration, frequency and design of training program) and in individual
features (gender, age and baseline physical status). The aim of this review article is to investigate the acute effects on thyroid hormones parameters that have been studied in endurance athletes.
Methods:Using PubMed database, review articles that discuss the influence of endurance physical activity on a thyroid hormone metabolism and hypothalamic-pituitary-thyroid axis.
Results:Eight studies have been elected. GALBO ET AL showed, with eight men during graded maximal and prolonged exhaustive treadmill running, an increased TSH concentration, but no
rise in concentrations of T3 and T4, after prolonged exercise compared to the rest. HESSE ET AL studied the effect of three distances of 75 km, 45 km and marathon (42.2km). T4 levels increased in the
75km and marathon group but decreased in the 45 km group post-race. T3 also dropped only in the 45 km group. rT3, measured only in the marathon and 75 km groups, rose in both groups. MALARKEY ET AL
investigated 35 triathletes and had a rise in TSH, which returned to normal over 18h post-event as a result. DESSYPRIS ET AL determined TSH and other hormones concentration before and after marathon,
in 10 runners, and found none of the functional parameters of the thyroid-pituitary axis significant changes. However, SANDER ET AL revealed an increase in TSH and fT4 post-marathon, with a decrease
in fT3 and rise in T4 to rT3 conversion, which was still detectable 22 hours after the race completion. SEMPLE ET AL report on marathon runners revealed no change in TSH, T4, T3, or rT3 levels before
and after the marathon. SCHUMANN ET AL saw no differences between the baseline and post concentration of serum TSH, in 29 subjects on a cycle ergometer over 30 minutes of steady-state cycling. HACKNEY
ET AL compared the thyroid hormonal responses to high-intensity interval exercise and steady-state endurance exercise in highly trained males and found increased levels of fT4, fT3 and rT3 after
Conclusions:In summary, the thyroid function changes secondary to exercise represent a complex physiologic response, which is difficult to characterize fully. There are many influence
factors in the thyroid hormones response to exercise. Results about the acute adaptation on thyroid hormones by endurance exercise are still conflicting. One of the most consistent findings is that
rT3 tends to increase with ultradistance exercise. TSH, T4 and fT4 seems to increase after endurance exercise. T3 appears to be unaffected by exercise. In conclusion, future studies should be
performed to try to isolate the acute influence of endurance exercise in thyroid hormones.