How could hyperthyroidism affect cardiac output?

Usually increases it: Cardiac output describes the amount of blood pumped through the heart in 1 minute. Two effects of high thyroid levels are an increased heart rate (more beats per minute) and an increased stroke volume (more blood per beat). These combine to increase the the cardiac output.. However, at some point, the increased work can cause the heart to fail, and the cardiac output will fall.

Hyperthyroidism: Hyperthyroidism is a excess metabolic state. If other systems are normal it will usually cause increased heart rate and cardiac output. In the face of a weak heart, it could be associated with decrease in cardiac output. Chronic hyperthyroidism can be associated with a cardiomyopathy causing decreased heart function.