Subclinical hypothyroidism (SCH) is characterized by elevated serum thyroid-stimulating hormone (TSH) levels and normal free thyroxine levels. The Korean Thyroid Association recently issued guidelines for managing SCH. Based on the Korea National Health and Nutrition Examination Survey (2013-2015), a serum TSH level of 6.8 mIU/L is the reference value for SCH. SCH is classified as mild (TSH 6.8-10.0 mIU/L) or severe (TSH > 10.0 mIU/L), and patients are categorized as adult (age < 70 years) or elderly (age ≥ 70 years). Levothyroxine treatment (LT4-Tx) is not recommended for mild or even severe SCH in elderly patients. Immediate LT4-Tx can be given to adults in most cases, but not to women who are pregnant, patients with progressive disease, or patients with underlying coronary artery disease, heart failure, or dyslipidemia. (Korean J Med 2024;99:127-133)