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Equine Vet J Suppl. 2006 Aug;(36):32-6.

Thyroid hormone responses to endurance exercise.

Author information

1
Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, Michigan 48824-1314, USA.

Abstract

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 (FT4) concentration after the 56 km ride. In contrast, total thyroxine (T4), total triiodothyronine (T3), FT4 and free triiodothyronine (FT3) 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 T4 concentration was lower (P<0.05) and FT3 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 T4, T3 and reverse triiodothyronine (rT3) 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 FT4 to 53% for FT3, 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.

[Indexed for MEDLINE]

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