Monitoring patients on thyroid replacement therapy Confirmation of thyroid-stimulating hormone (TSH) suppression in thyroid cancer patients on thyroxine therapy Prediction of thyrotropin-releasing hormone-stimulated TSH response An aid in the diagnosis of primary hyperthyroidism Differential diagnosis of hypothyroidismThe test is done to assess the level of FT4, It detects the biologically active fraction in circulating blood. Free T4 is the metabolically active fraction of thyroxine. FT4 along with TSH gives an accurate status of thyroid function in patients. This assay is useful for diagnosing both Hypo / Hyperthyroidism.