The screening serum thyroxine (T4) was compared to the history and physical examination as a means of detecting new cases of thyroid disease in a university hospital outpatient population. Of 13 new cases of hyperthyroidism diagnosed during the study period, 10 were suspected on the basis of history and physical examination, and 3 as a result of an elevated screening T4. Of 32 new cases of hypothyroidism, 23 were discovered on the basis of history and physical examination, and 9 as a result of a low screening T4. Thus approximately one quarter of new diagnoses of thyroid disease were made not by history and physical examination but by the unexpected appearance of a deviant T4 on the screening laboratory panel. Since approximately 40 percent of the deviant T4 values were not followed up by physicians, it is possible that an even higher yield of new cases would have resulted by laboratory detection alone. These results suggest that a significant number of new cases of thyroid disease are missed by the clinician and are uncovered only as a result of screening laboratory procedures.