1. Laboratory diagnosis of subclinical hypothyroidism: only TSH is increased, FT4 and FT3 are normal; 2. Main hazards of subclinical hypothyroidism: abnormal lipid metabolism and atherosclerosis, and may develop into clinical hypothyroidism; 3. Treatment of subclinical hypothyroidism: mainly thyroid hormone replacement therapy; 4. Additional knowledge: In 2004, the American Thyroid Association (ATA), American Association of Clinical Endocrinologists (AACE) (AACE) and the American Society of Endocrinologists (TES) reached the following consensus: (1) TSH>10mIU/L: L-T4 replacement therapy is advocated. The goals and methods of treatment are consistent with clinical hypothyroidism. (2) TSH between 4.0-10mIU/L: L-T4 therapy is not advocated and TSH changes are monitored regularly. (3) Patients with TSH 4-10mIU/L and positive TPOAb should be closely monitored for changes in TSH, as these patients are prone to develop clinical hypothyroidism.