Hypothyroidism is a common clinical entity which is often complicated by dyslipidemia. It is also reported increased risk for incidence of atherosclerosis and resulting coronary heart disease(CHD), heart failure(HF) and cardiovascular(CV) death. The effect of L-thyroxine replacement treatment on serum lipid and atherosclerosis is controversial in hypothyroid patients, especially in those with mild or moderate subclinical hypothyroidism. The present study was designed to investigate whether L-thyroxine replacement was effective in improving serum lipid profiles and retarding atherosclerosis progress.

Studies have shown that hypothyroidism increased the risk of COVID-19 composite poor outcomes. This study also aimed to investigate whether L-thyroxine replacement therapy was effective in reducing the incidence and mortality of COVID-19, and in improving the severity of COVID-19 and COVID-19 related complications.

Inclusion Criteria:

• 40~75 years old
• Diagnosis of overt or subclinical hypothyroidism in two occasions with a minimum interval period of three months

Exclusion Criteria:

• Pregnant or lactating women
• Severe hepatic or renal dysfunction
• Psychiatric disabilities, acute cardiovascular and cerebrovascular diseases, chronic respiratory diseases, familiar hypercholesterolemia, malignancy, cholelithiasis, pancreatitis, bowel diseases and other disorders influencing lipid and bile acid metabolism
• Taking lipid-lowering agents and other drugs influencing thyroid function, lipid and bile acid metabolism
• Obviously poor compliance.