Subclinical thyroid disease represents an early stage of thyroid dysfunction, which is usually asymptomatic and biochemically defined; its diagnosis can be performed thanks to the high sensitivity of the hypothalamic-pituitary-thyroid axis. The approach to this disorder requires correct diagnosis, clinical assessment and treatment. Cardiovascular diseases (e.g. atrial fibrillation, heart failure, and coronary heart disease), bone loss and fractures, and dementia represent the main adverse events of severe subclinical hyperthyroidism with undetectable TSH levels. Treatment of patients with subclinical hypothyroidism with a serum TSH level above 10 mIU/L is justified in order to reduce the risks of coronary heart disease and heart failure.