Subclinical hyperthyroidism (SHy), the mildest form of hyperthyroidism, is diagnosed in patients having a persistently low or undetectable serum concentration of thyroid-stimulating hormone (TSH) with normal free T4 and T3 concentrations. Although overt hyperthyroidism is associated with an increased risk of adverse cardiovascular outcomes, the cardiovascular risk of SHy is controversial. Multiple studies have demonstrated an increased risk of atrial fibrillation, especially in older individuals with TSH levels <0.1 mU/L. The effects of SHy on all-cause and cardiovascular mortality are not clear, but recent meta-analyses suggest a modest increase in mortality, with the risk increasing with age and associated with the lowest TSH levels. The long-term consequences of SHy in young- and middle-aged adults, and in those with TSH levels are mildly low, are uncertain. For these reasons, guidelines for treatment are based on patient age, the degree of TSH suppression, symptoms consistent with hyperthyroidism, and overall cardiovascular and osteoporotic fracture risks.