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Thyroid hormone responses to endurance exercise
Author(s) -
GRAVES E. A.,
II H. C. SCHOTT,
MARTENIUK J. V.,
REFSAL K. R.,
NACHREINER R. F.
Publication year - 2006
Publication title -
equine veterinary journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.82
H-Index - 87
eISSN - 2042-3306
pISSN - 0425-1644
DOI - 10.1111/j.2042-3306.2006.tb05509.x
Subject(s) - triiodothyronine , medicine , endocrinology , reverse triiodothyronine , endurance training , free thyroxine , hormone , thyroid , treadmill , horse , chemistry , biology , thyroid function , paleontology
Summary Reasons for performing study: Limited information exists about changes in circulating thyroid hormone concentrations during prolonged endurance exercise in horses. Objective: To examine the effects of prolonged exercise on serum iodothyronine concentrations in horses performing endurance exercise of varying distances. Methods: Serum concentrations of iodothyronines were measured in horses before and after completion of 40, 56, 80 and 160 km endurance rides (Study 1); daily during a 5 day, 424 km endurance ride (Study 2); and before and for 72 h after completion of a treadmill exercise test simulating a 60 km endurance ride (Study 3). Results: In Study 1, 40 and 56 km of endurance exercise had little effect on serum iodothyronine concentrations with the exception of a 10% decrease (P<0.05) in free thyroxine (FT 4 ) concentration after the 56 km ride. In contrast, total thyroxine (T 4 ), total triiodothyronine (T 3 ), FT 4 and free triiodothyronine (FT 3 ) concentrations all decreased (P<0.05) after successful completion of 80 and 160 km rides, with decreases ranging from 13–31% and 47–54% for distances of 80 and 160 km, respectively. Further, pre‐ride T 4 concentration was lower (P<0.05) and FT 3 concentration was higher (P<0.05) in horses competing 160 km as compared to horses competing over shorter distances. In Study 2, serum concentrations of T 4 , T 3 and reverse triiodothyronine (rT 3 ) progressively decreased (P<0.05) over the course of the multi‐day ride. In Study 3, the greatest decrease (P<0.05) in all iodothyronines was observed at 12 h of recovery, ranging from 25% for FT 4 to 53% for FT 3 , but all thyroid hormone concentrations had returned to the pre‐exercise values by 24 h of recovery. Conclusion: Endurance exercise results in transient decreases in serum iodothyronine concentrations. Potential relevance: These data are important to consider when thyroid gland function is assessed by measurement of serum iodothyronine concentrations in endurance horses.