The most appropriate diagnostic test to perform in this young patient with new-onset heart failure is to obtain thyroid studies. This patient exhibits signs and symptoms consistent with a diagnosis of hyperthyroidism, including tachycardia, a hyperdynamic precordium, palpitations, weight loss, and loose stools. Hyperthyroidism is a well-described, reversible cause of heart failure due to cardiac overstimulation by excess thyroid hormone that resembles sympathetic stimulation. Hyperthyroidism causes an increase in heart rate and myocardial contractility; systemic vascular resistance often decreases and may result in a widened pulse pressure. Hypothyroidism is also a known cause of heart failure, although it would be less likely in this patient with symptoms more consistent with excess thyroid hormone. Because thyroid function abnormalities are a potentially reversible cause of heart failure, assessment of thyroid function should be considered in patients with new-onset heart failure and clinical findings suggestive of thyroid dysfunction.

Evaluation of unusual causes of heart failure should not be performed routinely but should be pursued when there are suggestions of specific diseases by history or physical examination. The patient has no signs or symptoms suggesting a rheumatologic disorder, and routine screening with an antinuclear antibody level is not indicated. Similarly, this patient does not have a history of flu-like symptoms suggesting a viral etiology, making the potential yield of viral titers quite low. Furthermore, directed treatment options in the presence of positive viral titers are quite limited.

Endomyocardial biopsy is rarely indicated in the evaluation of acute heart failure as it is invasive and is unlikely to be helpful in identifying a reversible cause. It may be considered in patients whose heart failure is unresponsive to medical therapy or is associated with ventricular arrhythmias or conduction block in order to evaluate for giant cell myocarditis.