Which lab test should be performed before initiating treatment for bipolar disorder due to its potential influence on lithium therapy?

Before initiating treatment for bipolar disorder, particularly with lithium therapy, assessing thyroid function is crucial. Lithium has been known to affect the thyroid gland, potentially leading to hypothyroidism. Patients with bipolar disorder are at a higher risk for thyroid dysfunction, which can present with depressive symptoms that may complicate the clinical picture. Thus, a thyroid function test can help establish a baseline thyroid status and guide future monitoring to ensure that any changes in thyroid function during treatment are appropriately managed.

In the context of the other options, while renal function tests are important for monitoring patients on lithium due to the drug's renal clearance, they are more commonly assessed regularly during treatment rather than as a prerequisite before starting therapy. A CBC with differential is typically used to check for blood dyscrasias but isn’t specific to lithium therapy. A pregnancy test is crucial when prescribing certain medications that could affect fetal development, but in the context of lithium, it does not directly influence the initiation of therapy as thyroid function would. Therefore, the thyroid function test stands out as the most relevant lab test to perform beforehand.