Final answer:
A TSH test should be ordered for the patient on lithium to check for hypothyroidism as her symptoms align with those caused by this condition. Lithium is known to affect thyroid function, and appropriate monitoring is critical in managing her care.
Step-by-step explanation:
The 48-year-old female patient who is feeling tired and cold may be experiencing symptoms of hypothyroidism, a known side effect of lithium therapy. Given these clinical symptoms, the most appropriate test for the nurse practitioner to order would be a TSH (Thyroid-Stimulating Hormone) level. Hypothyroidism can cause symptoms such as fatigue, cold intolerance, weight gain, and a low heart rate. Since the patient is on lithium, it is important to monitor her thyroid function, as lithium can affect the thyroid gland's ability to produce hormones.
The patient's symptoms align with those described in Figure 12.5.6, which include fatigue and coldness, as well as other signs of hypothyroidism. Therefore, testing her TSH levels would be a logical step in assessing whether her symptoms are due to a thyroid issue potentially induced or exacerbated by her increased dose of lithium.