Subclinical thyrotoxicosis is defined as a below normal thyrotropin (TSH) in association with a normal total and free thyroxine (T4) and triiodothyronine (T3). It may be caused by thyroid hormone treatment or by endogenous thyroid disease. The degree of thyrotoxicosis may be estimated by the level of TSH suppression. Subclinical thyrotoxicosis may be associated with changes in cardiac performance and morphology, but this has not been consistently found in all patient populations. Changes may include increased heart rate, increased left ventricular mass index, increased cardiac contractility, diastolic dysfunction, and the induction of ectopic atrial beats or arrhythmias. Cardiac exercise performance may be impaired. Subclinical thyrotoxicosis should be treated in patients with cardiac symptoms or disease. Treatment may include reduction of the thyroid hormone dose, treatment of the underlying thyroid condition, or β-blockers.