 Background: 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. Subclinical hypothyroidism SHypo, defined as elevated serum TSH levels with normal thyroxine values, is common in older adults 5-10. 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 largest trial to date: 45 participants. Imaging endpoints are especially well-suited for early trials with investigational therapies for HF treatment and prevention, as well as validated and strong surrogate markers of clinical outcomes. 