Kamla, age 48 yrs, presents with unilateral mild axial proptosis. There is no redness or pain. Investigation of choice is:

All of the above conditions can cause proptosis. But, Graves' ophthalmopathy is the most common cause of unilateral or bilateral proptosis in adults or children. Orbital inflammation and engorgement of the extraocular muscles, paicularly the medial rectus and the inferior rectus, account for the protrusion of the globe. The accompanying upper eyelid retraction, manifesting as dispropoionately greater exposure of sclera superiorly than inferiorly, and lid lag, manifesting as impaired descent of the upper eyelid on downward gaze, distinguishes it from other causes of proptosis. Ref: Sullivan J.H. (2011). Chapter 13. Orbit. In P. Riordan-Eva, E.T. Cunningham, Jr. (Eds), Vaughan & Asbury's General Ophthalmology, 18e.