A lady presents with right knee swelling. Aspiration was done in which CPPD crystals were obtained. Next best investigation is -

Finding CPPD crystal in aspirated synovial fluid is diagnostic of CPPD arthropathy. CPPD arthropathy in some patients may be associated with :- Primary hyperparathyroidism, Hypothyroidism, Hemochromatosis, Hypomagnesemia, Hypophosphatasia. ANA (antinuclear antibody) is for SLE, RF (rheumatoid factor) is for rheumatoid arthritis and CPK (creatinine phosphokinase) is raised in dermatomyositis & polymyositis. So, among the given option only TSH is the relevant investigation in context of CPPD arthropathy as it may be associated with hypothyroidism.