Subclinical Hypothyroidism: Who Should Be Treated and Who Can Be Observed?

Subclinical hypothyroidism (SCH), defined by an elevated thyroid-stimulating hormone (TSH) concentration with a normal free thyroxine (FT4), is common, yet whether to treat it remains debated.This narrative review examines the natural history, clinical consequences, and treatment evidence for SCH and proposes a practical approach for identifying patients who should be treated and those who can be reasonably observed.PubMed/Medical Literature Analysis and Retrieval System Online (MEDLINE) and the Cochrane Library were searched from inception between September 1 and October 3, 2026 (last search), supplemented by referencelist screening; English-language adult studies and major guidelines were prioritized, with emphasis on randomized trials, systematic reviews, meta-analyses, and large prospective cohorts.SCH frequently resolves spontaneously or remains stable, particularly when TSH is below 10 mIU/L and thyroid peroxidase antibodies (TPOAb) are negative.Persistent TSH ≥10 mIU/L and TPOAb positivity predict progression to overt hypothyroidism, and coronary heart disease risk is clearly increased only at TSH ≥10 mIU/L, with no excess at 4.5-6.9mIU/L.Randomized trials, conducted mostly in older adults with mild elevation, show that levothyroxine normalizes TSH without improving symptoms, quality of life, or cardiovascular outcomes.Guidelines differ, mainly for TSH below 10 mIU/L.Persistent TSH ≥10 mIU/L generally favors treatment, whereas many patients with milder elevation, especially older adults, can be observed with scheduled monitoring.Pregnancy and preconception require a separate, timing-based approach.Decisions should be individualized rather than based on a single TSH cutoff.The proposed algorithm synthesizes existing evidence and guidelines and has not been prospectively validated.It should support, not replace, clinical judgment.