One of the most frequently performed operations in small animal clinics is the removal of testicles in male animals. As a result of this operation, the behavior and metabolic activities of the animals may change more or less depending on the changes in the sex hormone activities. Thyroxine hormone secreted from the thyroid gland is the main hormone that regulates the metabolic rate of the body. It has been reported that the risk of hypothyroidism in dogs is increased with sterilization in some studies (Milne and Hayes 1981, Panciera 1994) and not associated with sterilization in other studies. In this study, 24 male healthy dogs aged 2-3 years and weighing an average of 20 kg were used. Blood samples were taken from 24 healthy male dogs (Group 1) and their serums were extracted without any surgical intervention. One month after castration, blood samples were taken from all animals again and the serums were extracted (Group 2). TSH level was measured in animals in all groups using the canine ELISA kit (BT lab canine thyroid stimulating hormone ELISA test kit). The effect of castration on TSH was found to be statistically insignificant. Keywords: Castration, dog, TSH