The thyroid and the cardiovascular system are closely related, both in 
physiological and pathological conditions. The adverse consequences on the heart 
of overt thyroid disease are well-known and even subclinical forms of both 
hyperthyroidism and hypothyroidism are associated with increased cardiovascular 
mortality. In recent years, attention has shifted towards milder forms of 
thyroid disease, such as the so-called "low T3 syndrome", which is characterized 
by an isolated reduction in circulating levels of the biologically active form 
of thyroid hormone, triiodothyronine (T3). Furthermore, variations of T3 within 
the physiological range have been linked to coronary artery disease, one of the 
leading causes of morbidity and mortality worldwide. The present manuscript 
provides an overview of thyroid physiology and pathophysiology, with a 
particular focus on cardiovascular disease in patients with milder forms of 
thyroid dysfunction.