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.