Plasma thyroid hormone concentrations in dogs competing in a long-distance sled dog race.

Plasma thyroxine (T4), 3,5,3'-triiodothyronine (T3), total protein, and albumin concentrations were measured in 15 dogs both before and after completion, and in an additional 16 dogs before and 24 dogs after completion, of a long-distance sled dog race. The plasma T4 concentration (mean +/- SD) decreased significantly from 18.2 +/- 5.4 nmol/L before to 14.3 +/- 3.5 nmol/L after the race in dogs evaluated at both times and decreased significantly from 21.8 +/- 10.5 nmol/L before to 15.8 +/- 4.9 nmol/L after the race in dogs sampled only before or only after the race. The mean plasma T3 concentrations in dogs measured twice decreased significantly from 1.20 +/- 0.48 nmol/L before to 0.74 +/- 0.42 nmol/L after the race, as well as in dogs measured either before (1.28 +/- 0.36 nmol/L) or after (0.69 +/- 0.28 nmol/L) the race, respectively. Plasma total protein and albumin concentrations decreased significantly after completion of the race. No significant change was noted in 4 control dogs that did not compete in the race and were tested during a similar time period. The plasma concentrations of T4 and T3 were lower than the normal reference range established for this laboratory in 23 and 39%, respectively, of Alaskan sled dogs tested before the race. Plasma thyroid hormone concentrations frequently are below normal in conditioned Alaskan sled dogs and are further reduced after prolonged submaximal exercise.