Every clinician knows the symptoms, signs, and laboratory abnormalities associated with overt hyperthyroidism and hypothyroidism. Both conditions are unequivocally associated with morbidity and, occasionally, mortality, and no one would dispute the importance of timely diagnosis and therapy. In contrast, subclinical hyperthyroidism and hypothyroidism have subtle clinical manifestations at most, and the importance of timely diagnosis and treatment continue to be contentious subjects of research studies, position papers, and editorials. The central issues are whether subclinical thyroid diseases are of sufficient clinical importance to warrant screening or case finding and whether, once these conditions are detected by an abnormal serum thyrotropin value and confirmed by further testing, therapy is justified. Because reliable thyroid-stimulating hormone (TSH) assays have been available for almost 20 years, one would think that adequate data would have been amassed by now to answer these questions definitively. However, this is not the case, and we continue to rely on small clinical studies, epidemiologic data, computer simulations , and clinical impressions rather than the results of large randomized clinical trials.