 Expected value of the proposed project: Controversies persist regarding the indications for screening and treatment of SHypo due to the lack of an appropriately powered RCT addressing the impact of thyroid replacement on CV outcomes. 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. The strengths of this study include the combination of: 1 High feasibility of the project in collaboration with the largest RCT on SHypo making it possible to investigate the mechanisms of associations with CV disease; 2 Innovative project with combined CV imaging to assess cardiac function with thyroid replacement therapy vs. placebo; 3 Excellent power given the large sample size and participants older age. The sample size for this proposal will be 5-fold higher than previous small trials of thyroid replacement on CV outcomes. The collaboration of CV imaging modalities expertise with the ongoing largest trial on SHypo, is a unique opportunity to address the clinical and scientific issue of the impact of SHypo on the CV system. 