Subclinical thyroid dysfunction, defined as normal thyroid hormone levels associated with abnormal thyrotropin (TSH) concentrations, that are called as subclinical hypo- (SH) or thyrotoxicosis (ST) indicates mild thyroid failure. Thus thyroid hormone levels are not truly normal for that individual. SH is often reported to be associated with unfavorable lipid metabolism. Some studies sug-gest patients with SH had a significantly increased prevalence of myocardial infarctions in the elderly. The common view recommends that SH be treated with L-thyroxine when TSH concentrations are 10μU/ml or more to avoid cardiovascular events. In patients with ST atrial fibrillation reportedly develops at a substantial frequency. A population-based cohort study showed that low TSH in older individuals is associated with increased mortality, especially due to cardiovascular disease. These findings suggest the beneficial effects of treating ST. The overall prevalence of high and low TSH concentrations reportedly range from 2-10% and 0.2-3%, respectively, in large general population screening surveys. Most thyroid dysfunction was subclinical, but some was overt, in which treatment is essential. Hypothyroidism is seen more in older people, and hyperthyroidism develops most commonly in the third and forth decades in Japan. The frequencies of hypo- and hyperthyroidism are higher in women. Thyroid dysfunction can be treated rather easily using inexpensive drugs. These facts may warrant the screening for thyroid dysfunction using TSH in apparently healthy adults, especially in older women because of higher incidence of SH. However, clinical data suggest that in thyrotoxic patients, men are more susceptible to cardiovascular dysfunction than women. Furthermore, in younger people, thyroid dysfunction may exert adverse effects on various life events, especially on pregnancy. So everyone is recommended to have TSH determination. Thyroid palpation and careful monitoring of changes in liver function and lipid profile, which are useful indicators of thyroid dysfunc-tion, are essential to detect it and reduce fruitless TSH examination.