As previously noted, some thyroidologists and laboratorians advocate screening for thyroid disease with a single test, often citing need for cost containment. The ultra sensitive TSH appears to be advantageous for this purpose. The T4 test result is more frequently normal in mild disease and more frequently abnormal in the absence of thyroid disease than is the ultra sensitive TSH. The FT4 test has many of the same problems as the T4 test, although the FT4 is less frequently affected by non thyroid conditions. The major problem with the one-test approach is that the clinician becomes very dependent on the laboratory to select a reliable method or commercial kit. I can verify that not all commercially available kits are equally reliable and that it takes search for published evaluations of the exact manufacturer’s kit under consideration in addition to extensive evaluation of the kit in the potential user’s own laboratory to establish proof of reliability. In addition, each laboratory should establish its own reference range using a statistically satisfactory number (at least 20) of blood donors or other clinically normal persons.

Finally, one has to consider the possibility of laboratory error, even though it may not be frequent on a statistical basis. For these and other reasons, some order two tests rather than one for screening purposes, such as the FT4 plus the ultra sensitive TSH. If both test results are normal, this is very reassuring. If one or both are abnormal, either or both can be repeated to verify abnormality. The diagnosis sometimes can be made with this evidence plus clinical findings, or additional tests may be needed.

Since the T4 (or its variants) and the ultra sensitive TSH are being used extensively to screen for and diagnose thyroid disease, it might be useful to catalog the patterns encountered using these two tests and some of the conditions that produce these patterns (see the box).