4 Diagnosis of hyperthyroidism: The newer biochemical tests

Investigation of suspected hyperthyroidism is conventionally based on measurement of total or free T₄ as the initial test, followed in equivocal cases by total or free T₃, and the TRH test. Recent developments in techniques for measuring free hormones and TSH promise to change this approach. Free T₄ and free T₃ can now be rapidly and simply quantitated in whole serum using labelled analogue radioimmunoassays. Specific and highly sensitive assays using labelled monoclonal antibodies are now available for serum TSH that permit the suppressed levels found in most cases of hyperthyroidism to be distinguished from euthyroid levels. These newer assays are available at a cost per test that is often similar to that of the more established tests.

Available evidence indicates that measurement of basal serum TSH by a sensitive labelled antibody method can serve as a first line test, at least in uncomplicated cases of suspected hyperthyroidism. In patients with a suppressed TSH, a serum free T₄, and in equivocal cases free T₃, will distinguish the clinical and subclinical forms of hyperthyroidism. Such an approach would obviate the need for the TRH test. It must be emphasized, however, that experience with this new approach is limited. Caution is advised in the interpretation of low TSH and free hormone levels when there are associated complicating features, such as severe non-thyroidal illness, or pregnancy.

These developments mark a trend in thyroid function testing away from measurement of circulating total hormone levels. The newer tests provide an assessment of end organ (thyrotroph) response, and an assessment of biologically active (free) hormone to which the tissues are exposed. These complementary approaches have the potential to identify relatively minor degrees of thyroid dysfunction.