A 29 year old smoker presents with dry eyes and diplopia. What is the SINGLE most appropriate next step in management?

In this clinical scenario, the symptoms of dry eyes and diplopia in a 29-year-old smoker raise suspicion for an autoimmune condition, particularly myasthenia gravis or possibly thyroid eye disease, given the patient's demographic and presentation.

Performing thyroid function tests is a critical first step because these tests can help rule out hyperthyroidism, which can be associated with Graves’ disease—a condition known to cause exophthalmos (protrusion of the eyes) and other ocular symptoms, as well as dry eyes due to thyroid dysfunction. Additionally, thyroid disorders can be linked to Graves' ophthalmopathy, which aligns with the symptoms described.

Identifying thyroid dysfunction early on can guide further management and treatment decisions, particularly since conditions like hyperthyroidism often require prompt treatment to prevent complications. The other proposed tests, while potentially useful in assessing the broader clinical picture, do not provide the immediate and relevant information about the patient's thyroid status, which could be impacting their ocular symptoms.