The definition of subclinical hypothyroidism (SCH) is solely biochemical: a serum free T4 level within the reference range in the presence of an elevated serum thyroid-stimulating hormone (TSH) level. While overt hypothyroidism is associated with an increased cardiovascular disease risk, SCH, the mildest form of hypothyroidism, may also be associated with an increased cardiovascular disease risk, but to a lesser degree. Recent evidence points to a significant trend toward an increase in cardiovascular risk at higher TSH levels, with TSH levels ≥10 mIU/L associated with increased cardiovascular morbidity and mortality. It is generally recommended to treat with thyroid hormone those individuals with SCH and TSH values ≥10 mIU/L. Treatment of patients with SCH and TSH values <10 is controversial but may be considered in selected patients.