Objective—To determine effects of athletic conditioning
on thyroid hormone concentrations in a population
of healthy sled dogs.
Animals—19 healthy adult sled dogs.
Procedure—Serum concentrations of thyroxine (T4),
triiodothyronine (T3), thyroid-stimulating hormone
(TSH), free T4 (fT4), free T3 (fT3), and autoantibodies
directed against T3, T4, and thyroglobulin were measured
in sled dogs that were not in training (ie, nonracing
season) and again after dogs had been training
at maximum athletic potential for 4 months.
Results—Analysis revealed significant decreases in
T4 and fT4 concentrations and a significant increase in
TSH concentration for dogs in the peak training state,
compared with concentrations for dogs in the
untrained state. Serum concentrations of T4 and fT4
were less than established reference ranges during
the peak training state for 11 of 19 and 8 of 19 dogs,
respectively; fT4 concentration was greater than the
established reference range in 9 of 19 dogs in the
Conclusions and Clinical Relevance—Decreased
total T4 and fT4 concentrations and increased serum
concentrations of TSH were consistently measured
during the peak training state in healthy sled dogs,
compared with concentrations determined during the
untrained state. Although thyroid hormone concentrations
remained within the established reference
ranges in many of the dogs, values that were outside
the reference range in some dogs could potentially
lead to an incorrect assessment of thyroid status.
Endurance training has a profound impact on the thyroid
hormone concentrations of competitive sled
dogs. ( Am J Vet Res 2004;65:333–337)