The existence of deceptive laboratory hyperthyroidism with the various forms of pseudo toxicosis accentuates the need for reliable confirmatory tests. This is especially true when the patient has severe non thyroidal illness and symptoms such as atrial fibrillation that may be due to thyrotoxicosis. At present, the two most useful confirmatory procedures for hyperthyroidism are the T3 suppression test and the TRH test. Of these, the main advantage of T3 suppression over TRH is lower cost. Major disadvantages include potential danger in persons with cardiac disease and the prolonged time period necessary for the test. The TRH test has emerged as the gold standard for diagnosis of hyperthyroidism. The procedure appears to be safe, with relatively minor side effects. It can be completed in less than 1 day and can be used in most (but not all) patients with non thyroidal illness. The major disadvantage is that a small minority of persons without clinical or biochemical evidence of thyrotoxicosis are reported to demonstrate TRH test results compatible with hyperthyroidism. A relative disadvantage is the high cost of the test, although the cost is not prohibitive and is comparable to the cost of nuclear medicine scans or radiologic procedures such as laminograms or skeletal surveys.
From the preceding discussion, several conclusions seem warranted:

1. The basic screening test for hyperthyroidism is serum T4 or FT4. Some are using the newer ultra sensitive TSH assay instead.
2. The THBR when ordered with the T4 may be useful for two reasons: (1) the assurance it provides for hyperthyroidism when the THBR value is elevated in association with an elevated T4 value and (2) as an indicator of TBG alterations when the results are compared with those of T4 assay.
3. Free thyroxine index or FT4 assay provide T4 values corrected for effects of TBG alterations; but if the TBG value is elevated and thereby produces falsely elevated T3-RIA values, a normal FT4I or FT4 assay result may lead to misdiagnosis of T3 toxicosis.
4. Decreased or mildly elevated T4 results (or even decreased TI or FT4 results, depending on the individual commercial kit) must be interpreted with caution when a patient has severe non thyroid illness.
5. The T3-RIA results are not reliable when a patient has moderate or severe non thyroid illness. A normal or reduced T3-RIA result must be interpreted with caution if the patient is over age 60 or has any significant degree of non thyroid illness.
6. An RAIU test is not recommended as a screening test but is helpful as a follow-up procedure to detect factitious hyperthyroidism and thyroiditis.
7. Thyroid scan is useful to differentiate between Graves’ disease and Plummer’s disease and to help reveal thyroiditis.
8. The TRH test may be necessary to confirm or exclude the diagnosis of hyperthyroidism when the patient has non thyroidal illness (not severe enough to decrease T4 below reference range), when thyroid function tests disagree, when test results are equivocal, and when test results do not fit the clinical picture. A normal test result is reasonably conclusive in ruling out hyperthyroidism, whereas an abnormal test result (abnormally low TSH response) is suggestive evidence of hyperthyroidism but is not completely reliable in confirming the diagnosis.