What is the first step in the diagnostic evaluation of a patient with suspected thyroid dysfunction during lithium therapy?

The initial step in assessing a patient on lithium therapy for suspected thyroid dysfunction is measuring serum TSH (thyroid-stimulating hormone) levels. Lithium use is associated with both hypothyroidism and goiter, making thyroid function monitoring essential in these patients. TSH serves as a sensitive marker of thyroid function; an elevated TSH indicates that the thyroid gland is not producing adequate amounts of thyroid hormones, which can help confirm a diagnosis of hypothyroidism.

In the context of patients on lithium, it is crucial to routinely assess thyroid function, especially since changes in thyroid status can occur relatively quickly after starting lithium therapy. This proactive approach allows for timely interventions if thyroid dysfunction is detected.

Other potential steps, such as assessing serum calcium, checking liver enzymes, or reviewing history for adverse effects, may provide valuable information in different clinical circumstances but do not specifically address the immediate need to evaluate the thyroid's response to lithium. Hence, measuring serum TSH is the most pertinent first step in this scenario.