A 41-year-old female on lithium therapy presents with a routine TSH level of 4.5. What is the most appropriate action?

In the context of lithium therapy, thyroid function monitoring is crucial because lithium can affect thyroid hormone levels, often leading to hypothyroidism. The normal range for TSH levels is typically around 0.4 to 4.0 mIU/L; thus, a TSH level of 4.5 indicates a slight elevation that may suggest subclinical hypothyroidism.

Beginning thyroid replacement therapy would be a proactive measure to manage this potential thyroid dysfunction. This is especially important as the patient is on lithium, which has a known association with inducing hypothyroidism in some individuals. Administering thyroid replacement therapy can help normalize thyroid hormone levels, thereby potentially alleviating any related symptoms and avoiding further complications.

Continuing the current treatment might overlook the elevated TSH and its implications for the patient's overall health. Increasing the dose of lithium is not a suitable choice, as it would not address the thyroid function concern. Referring to an endocrinologist could be considered in complex cases, but in this instance, initiating thyroid replacement therapy is the most appropriate immediate action to address the mild elevation of TSH.