Hyperthyroidism has profound effects on the cardiovascular system, including reduced systemic vascular resistance (SVR) due to relaxation of vascular smooth muscle cells, enhanced heart rate (HR) and cardiac output (CO) due to an increase in cardiac diastolic relaxation, contractility and heart rate. Subclinical hyperthyroidism is characterised by reduced serum TSH levels despite free thyroxine (T4) and tri-iodothyronine (T3) estimates within the reference range, in subjects with no obvious symptoms of hyperthyroidism. We measured haemodynamic changes (using impedance cardiography) in subjects with endogenous subclinical hyperthyroidism in order to elucidate whether these patients had signs of excess thyroid hormone at the tissue level.