What is the most likely cause of decreased exercise tolerance in a dog that has a history of hypothyroidism and a racing heart rate?

Decreased exercise tolerance in a dog with a history of hypothyroidism and a racing heart rate is most likely attributed to decreased cardiac output. Hypothyroidism can lead to alterations in metabolism and cardiovascular function, and a racing heart rate often indicates that the heart is trying to compensate for inadequate blood supply to meet the body's demands. When the heart cannot pump effectively or enough blood is not being circulated, this can result in reduced oxygen delivery to the tissues during physical activity, thus leading to exercise intolerance.

Additionally, hypothyroidism may already predispose the dog to cardiovascular issues, affecting overall heart performance. With a combination of a history of thyroid dysfunction and the observed symptoms, decreased cardiac output emerges as a prominent factor contributing to the animal's inability to tolerate exercise.