Thyroid hormone stimulates cardiac inotropy and chronotropy via direct genomic and non-genomic mechanisms. Hyperthyroidism magnifies these effects, resulting in an increase in heart rate, ejection fraction and blood volume. Hyperthyroidism also affects thrombogenesis and this may be linked to a probable tendency towards thrombosis in patients with hyperthyroidism. Patients with hyperthyroidism are therefore at higher risk for atrial fibrillation, heart failure and cardiovascular mortality. Similarly, TSH suppressive therapy for differentiated thyroid cancer is associated with increased cardiovascular risk. In this review, we present the latest insights on the cardiac effects of thyroid suppression therapy for the treatment of thyroid cancer. Finally, we will show new clinical data on how to implement this knowledge into the clinical practice of preventive medicine.