This chapter reviews the data on subclinical thyroid dysfunction and heart failure and mortality. Observational studies have suggested that subclinical hypothyroidism is associated with a moderately elevated risk of heart failure events among older adults with a thyrotropin level higher than 7–10 mU/L. Such data are lacking for middle-aged adults. Subclinical thyroid dysfunction might also represent a potentially modifiable risk factor for mortality. In a recent systematic review and meta-analysis of prospective cohorts, adults less than 65 years of age with subclinical hypothyroidism were found to be at increased risk of all-cause and cardiovascular mortality. The situation might be reversed in the elderly. Data are more limited for subclinical hyperthyroidism. Large, randomized clinical trials are needed to assess the efficacy of thyroxine replacement or antithyroid medications on clinical outcomes such as cardiovascular disease (coronary heart disease, heart failure, cardiovascular mortality) in adults with subclinical thyroid dysfunction.