<qa>  What is the leading cause of mortality and morbidity?Coronary heart disease CHD and heart failure HF are the leading causes of mortality and morbidity, particularly with the current context of an aging population. Prospective cohort studies, as well as analyses of pooled individual participant data suggest up to a 60-90 increase in the risk of CHD or HF events among adults with severe SHypo. However, no large randomized controlled trials RCT have assessed the impact of thyroid replacement on cardiovascular CV imaging outcomes. The goals of this proposal are to address the impact of thyroid replacement on cardiac function. The investigators will conduct a RCT in 185 patients with subclinical hypothyroidism who will be randomly assigned to thyroxine or placebo with an average follow-up of 24 months from baseline. The main outcome will be CV imaging modalities measured by echocardiography at the close-out visit. Assessment of the impact of thyroid replacement on cardiac function and subclinical atherosclerosis within a trial will aid decisions and evidence-based guidelines development to treat a potential modifiable risk factor, such as SHypo. 