Thyroid hormones play an important role in regulating the cardiovascular system. Thyroid dysfunction, which encompasses hypothyroidism and hyperthyroidism, is relatively common and has been linked to a range of cardiovascular manifestations, including dyslipidaemia, abnormal blood pressure regulation, endothelial dysfunction, impaired cardiac function, and arrhythmias. The effects of levothyroxine replacement therapy on cardiovascular endpoints in subclinical hypothyroidism remain uncertain. The largest randomized controlled trial (TRUST) to date was underpowered to assess cardiovascular outcomes, although the results demonstrated a pattern towards a beneficial effect of levothyroxine. The aim of this review was to provide a summary of current knowledge on the relationship between thyroid dysfunction and cardiovascular health, with a focus on molecular and pathophysiological mechanisms underlying thyroid dysfunction and cardiovascular disease. Evidence from recent omics and Mendelian randomization studies is discussed, alongside an overview of the epidemiology of thyroid diseases and the impact of levothyroxine treatment on cardiovascular outcomes in subclinical hypothyroidism.