Thyroid hormones are essential for human survival, and suboptimal levels of thyroid hormones cause progressive systemic decline and increased mortality risk.

Study: Risk factors and clinical implications of thyroxine replacement therapy on major adverse cardiovascular events in type 2 diabetes: a retrospective cohort study

When people don’t have enough thyroid hormone, their body doesn’t work right — even when they take medicine to fix it, they’re still at higher risk for serious heart problems and other dangers. This study shows that low thyroid hormone levels are linked to worse health outcomes, meaning the body really needs enough of these hormones to survive.

Study: Supplementation with selenium and coenzyme Q10 in an elderly Swedish population low in selenium — positive effects on thyroid hormones, cardiovascular mortality, and quality of life

When people don’t get enough selenium, their thyroid doesn’t work well, which can make them sicker and more likely to die of heart problems. Giving them selenium helped their thyroid work better and lowered those risks.

Study: EVALUATING THYROID HORMONE INFLUENCE ON CARDIOVASCULAR RISK AMONG PATIENTS WITH SUBCLINICAL AND CLINICAL HYPOTHYROIDISM

When your thyroid doesn't make enough hormones, your heart and blood vessels start to struggle—this study shows that the lower the hormone levels, the worse your heart function and cholesterol get, which can make you more likely to get seriously sick or die.