Objectives-To determine the effects of racing and training on serum thyroxine (T4), triiodothyronine (T3), and thyroid stimulating hormone (TSH) concentrations in Greyhounds. Animals-9 adult racing Greyhounds. Procedure-Serum thyroid hormone concentrations were measured before and 5 minutes after a race in dogs trained to race 500m twice weekly for 6 months. Resting concentrations were measured again when these dogs had been neutered and had not raced for 3 months. Postrace concentrations were adjusted relative to albumin concentration to allow for effects of hemoconcentration. Thyroid hormone concentrations were then compared with those of clinically normal dogs of non-Greyhound breeds. Results-When adjusted for hemoconcentration, total T4 concentrations increased significantly after racing and TSH concentrations decreased; however, there was no evidence of a change in free T4 or total or free T3 concentrations. Resting total T4 concentrations increased significantly when dogs had been neutered and were not in training. There was no evidence that training and neutering affected resting TSH, total or free T3, or free T4 concentrations. Resting concentrations of T3, TSH, and autoantibodies against T4, T3, and thyroglobulin were similar to those found in other breeds; however, resting free and total T4 concentrations were lower than those found in other breeds. Conclusions and Clinical Relevance-Except for total T4, thyroid hormone concentrations in Greyhounds are affected little by sprint racing and training. Greyhounds with low resting total and free T4 concentrations may not be hypothyroid.