Subclinical hyperthyroidism is defined as abnormal low TSH level with thyroid hormones within their reference range. This laboratory condition may be symptomatic in a relevant number of patients leading to tachycardia, sweating, nervousness, anxiety and insomnia. The risk for cardiovascular disease is increased with more frequent atrial fibrillation and increased left ventricular mass including diastolic dysfunction. Cardiovascular mortality and overall mortality surmounts the average of the normal population. Longterm TSH suppression leads to decreased bone mineral density and an increased fracture rate in the hip and in the spine. After evaluation of underlying causes, therapy should be considered, especially if TSH levels are below 0.1 mIU/l.