The best screening method is measurement of the FT4 level using a robust assay. Some suggest a sensitive TSH assay as an initial screen, but low values have been reported in some normal subjects and in patients with severe non thyroidal illness. If the FT4 level is normal, the FT3 level should be checked if there is clinical evidence of hyperthyroidism. T3 toxicosis is seen in the early stages of development of hyperthyroidism, in relapse, and in iodine deficiency. The TRH test is now used only if the results of the other tests are equivocal.
