Which of the following is most likely to be decreased in an otherwise healthy patient with untreated hyperthyroidism?

The increased metabolic rate associated with hyperthyroidism has cardiovascular consequences. Tachycardia is the most commonly recognized circulatory change. An increase in nitric oxide production is associated with a decrease in systemic vascular resistance (SVR). The decreased SVR results in activation of the renin–angiotensin–aldosterone system, producing an increase in plasma volume. That, coupled with increased levels of erythropoietin, results in an increased blood volume. The direct inotropic effects of thyroid hormones increase ejection fraction, which, in combination with a decreased SVR, increases pulse pressure. Patients with untreated hyperthyroidism are likely to have an increased, not a decreased, cardiac output.

There is also an increased prevalence of dysrhythmias, predominantly supraventricular tachycardia. Studies have demonstrated that 28 percent of patients over 60 years old who have decreased levels of thyroid stimu-lating hormone developed atrial fibrillation....

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