Since many tests to investigate thyroid function are currently available, appropriate selection is required to limit the number of assay needed to establish the correct diagnosis of thyroid dysfunction. The limitations inherent in the different tests, and the interferences caused by nonthyroidal factors, especially drugs, must, therefore, be taken into account. Serum total thyroid hormone (TT4 and TT3) determinations are largely affected by changes in the concentrations of thyroid hormone transport proteins (mainly T4-binding globulin). Thus, in many cases, serum TT4 and TT3 measurements do not reliably establish thyroid status. Serum free thyroid hormone (FT4 and FT3) concentrations are independent of transport proteins and more appropriately reflect thyroid status. Serum FT3 measurement is more appropriate for the diagnosis of hyperthyroidism and drug-overdosage in L-T4-treated patients. Conversely, serum FT4 measurement more correctly identifies hypothyroid patients. Serum TSH determination by the currently available sensitive (low detection limit) assays constitutes an indispensable complementary test in both conditions.