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Subclinical hypothyroidism is defined by an elevated serum thyroid-stimulating hormone (TSH) level together with a normal free thyroxine (T4) level. There is controversy over whether it should be treated. Currently, the best practical approach is to base treatment decisions on the degree of TSH elevation, thyroid autoimmunity, and associated comorbidities. KEY POINTS: Between 4% and 20% of adults have subclinical hypothyroidism, with a higher prevalence in women, older individuals, and those with thyroid autoimmunity. Subclinical hypothyroidism may progress to overt hypothyroidism, especially if antithyroid antibodies are present, and has been associated with adverse metabolic, cardiovascular, reproductive, maternal-fetal, neuromuscular, and cognitive abnormalities, as well as decreased quality of life. Some studies have suggested that levothyroxine therapy is beneficial, but others have not, possibly due to variability in study designs, sample sizes, and patient populations. Further trials are needed to clearly demonstrate the clinical impact of subclinical hypothyroidism and the effect of levothyroxine therapy.