Background <p>Three Boxers, two neutered females and one neutered male, aged from seven to eleven years, were presented for investigation of transient loss of consciousness.</p> Case presentation <p>In each case, physical examination revealed no significant abnormalities and echocardiography, and electrocardiography did not identify a cause of transient loss of consciousness. Twenty-four-hour electrocardiogram (Holter) monitoring was performed in all three dogs. One dog had an episode of transient loss of consciousness whilst wearing the Holter, during which second-degree atrioventricular block and sinus bradycardia were observed. Based on clinicopathological testing, all three dogs were diagnosed with hypothyroidism. Once hypothyroidism was diagnosed all dogs were started on levothyroxine. In all cases the transient loss of consciousness resolved or improved once the treatment was started.</p> Conclusion <p>Hypothyroidism should be considered as a differential diagnosis in Boxers presenting with transient loss of consciousness. Treating hypothyroid Boxers may decrease or resolve the clinical signs.</p>