Hyperthyroidism is most often due to thyroid gland hyperfunction (primary hyperthyroidism) best diagnosed with measurement of T4, which will be high, when thyroid stimulatng hormone (TSH) is low. Measurement of T3 is rarely of value except when there is evidence of hyperthyroidism and the T4 is not high. Measurement of thyroid stimulating immunoglobulin (TSI) is helpful to confirm the diagnosis of Graves disease, and TSI levels can monitor the risk for thyroid eye disease and response to therapy. |