It is well known that exercise increases the activity of thyroid glands and raises the blood level of melatonin. The increase of melatonin during exercise may be linked to a rise in thyroid-stimulating hormone (TSH). No previous study has investigated the combined effects of melatonin ingestion and acute submaximal exercise on thyroid hormones' responses. The purpose of this pilot study was to explore the effects of daytime ingestion of melatonin on thyroid hormones' responses to acute submaximal exercise. After 50 min of either melatonin (6 mg) or placebo ingestion, eight physical education students (mean ± standard deviation of age: 22 ± 1 years) were asked to run for 45 min at 60% of their maximum aerobic speed. Free thyroxine (fT4) and TSH were measured in plasma samples before and immediately after exercise. After submaximal exercise, TSH increased by 54% in both placebo and melatonin conditions. There was no significant (Condition × Exercise) interaction, and no significant condition effect for TSH. The fT4 remained unchanged before/after submaximal exercise in both placebo [15.2 (1.9) and 15.0 (1.6) pmol/L, respectively, p > .05], and melatonin [16.7 (2.7) and 16.3 (2.7) pmol/L, respectively, p > .05] conditions. There was no significant (Condition × Exercise) interaction, no significant exercise effect, and no significant condition effect for fT4. To conclude, acute melatonin ingestion did not affect thyroid hormones' responses to submaximal exercise.
Keywords: biological rhythm; inflammation; metabolism; oxidative stress; thyrotropic axis.