The investigation of thyroid function.

The measurement of thyrotropin (TSH) by a third-generation assay (functional sensitivity 0.01-0.02 mU/L) offers the single best test of assessing thyroid status and is more cost effective than measurement of thyroxine (T4) as a first-line test. However, in certain situations, e.g., nonthyroidal illness (NTI), secondary thyroid disease, early treatment of hyperthyroidism, early pregnancy, and heterophilic antibody interference, TSH alone may be misleading, and thyroid hormone measurements are also required. The author's laboratory has found that the measurement of TSH together with free thyroxine (by a nonanalogue method) offers the most speedy and effective first-line strategy for thyroid function testing. Abnormalities in thyroid function tests in euthyroid patients are most likely found in NTI and the elderly. For this reason, thyroid function tests (TFTs) should not be requested in elderly or hospitalized patients unless the presenting complaint is considered to be because of a thyroid problem.