A 36-year-old female with bipolar I disorder has been taking lithium 1,200 mg daily for three years with excellent mood stability. At her routine six-month monitoring visit, her lithium level is 0.8 mEq/L and she is euthymic. However, her TSH has increased from 3.2 mIU/L six months ago to 7.8 mIU/L today, with a free T4 of 0.7 ng/dL (reference range 0.8-1.8 ng/dL). She reports increased fatigue over the past two months, mild constipation, and feeling colder than usual, but attributed these symptoms to seasonal changes and work stress. She has no personal or family history of thyroid disease. Her prior TSH values on lithium were consistently between 2.5 and 3.5 mIU/L. The PMHNP is evaluating these thyroid monitoring results in the context of ongoing lithium therapy. Which evaluation is most clinically sound?