Which of the following is the best marker for diagnosing thyroid related disorder?

Thyroid Stimulating Hormone assay is highly sensitive and specific in the laboratory assessment of thyroid function. TSH levels change dynamically in response to alterations of T4 and T3, so while evaluating a case of thyroid dysfunction, first we have to determine whether TSH is suppressed, normal or elevated. With rare exceptions a normal TSH level excludes a primary abnormality of thyroid function. The finding of an abnormal TSH level must be followed by measurements of circulating thyroid hormone levels to confirm the diagnosis of hypehyroidism or hypothyroidism. TSH should not be used as an isolated laboratory test to assess thyroid function in patients with suspected or known pituitary disease. Ref: Harrison's Internal Medicine, 18th Edition, Pages 2916-2917, Chapter 341 ; Principles of Critical Care, 3rd Eedition, Jesse B. Hall, Chapter 80.