From the preceding discussion, several conclusions seem warranted:

1. Serum T4 is the most widely used screening test for hypothyroidism, but some physicians use FT4 or serum TSH assay instead. Values in the upper half of the T4 and FT4 reference range are strong evidence against hypothyroidism unless the TBG level is increased (congenital, pregnancy, or estrogen induced).
2. The THBR test is useful to detect TBG-induced alterations in T4.
3. The new FT4 methods circumvent the majority of TBG-induced problems and significantly reduce the number of pseudo hypothyroid cases due to severe non thyroidal illness. However, even the FT4 may give falsely decreased results in seriously ill patients.
4. The TSH assay is the most useful single test to confirm primary hypothyroidism. In occasional patients a TRH test may be necessary.
5. Certain conditions may produce decreased T4 levels, increased TSH levels, or both in occasional patients without primary hypothyroidism.