In a patient with newly diagnosed atrial fibrillation, which test is most important to order to evaluate an underlying reversible cause?

• A. Electrocardiogram
• B. Thyroid-stimulating hormone level
• C. Echocardiography
• D. Coronary angiography

Answer: (B)

When someone is newly diagnosed with atrial fibrillation, it’s important to look for reversible triggers that, if found and treated, can improve or resolve the rhythm issue. Thyroid disease, particularly hyperthyroidism, is a well-known, reversible cause of AF. A thyroid-stimulating hormone level is the best initial screen because it’s highly sensitive for detecting thyroid dysfunction and is quick and inexpensive. If the TSH is abnormal, checking free T4 (and possibly T3) confirms hyperthyroidism and guides treatment; treating the underlying thyroid issue can reduce AF burden and sometimes restore normal rhythm.

The other tests have different roles: an electrocardiogram confirms the presence of AF but doesn’t identify reversible causes; echocardiography assesses structural heart disease and function; coronary angiography evaluates coronary anatomy. None of these specifically targets reversible thyroid-related triggers as effectively as a TSH test does.