Final answer:
Before administering lithium to a patient with bipolar disorder, a nurse should ensure that a thyroid hormone assay has been completed to monitor the medication's potential impact on thyroid function.
Step-by-step explanation:
A nurse caring for a client who has bipolar disorder and has been taking lithium for 1 year should check to see that a thyroid hormone assay has been completed before administering the medication. This test is important because lithium can affect thyroid function, potentially leading to hypothyroidism or hyperthyroidism. Monitoring thyroid function is an essential part of managing patients on long-term lithium therapy. Endocrinologists often use laboratory tests to diagnose endocrine disorders, and lithium's impact on the thyroid is one such concern that requires regular assessment via blood tests.