Association Between Metabolic Syndrome and Subclinical Hypothyroidism: A Systematic Review.

Metabolic syndrome (MetS) and subclinical hypothyroidism (SCH) are prevalent endocrine-metabolic disorders with significant cardiovascular implications. Emerging evidence suggests a bidirectional relationship between these conditions, but findings remain inconsistent across populations. This systematic review synthesizes existing literature to evaluate the association between SCH and MetS, focusing on prevalence, demographic variations, and mechanistic links. Following Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines, a comprehensive search was conducted across PubMed, Scopus, Web of Science, and Embase up to April 2025. Twelve observational studies (cross-sectional, cohort, and case-control) were included after screening. Studies were selected based on predefined criteria: adult populations (≥18 years), clear definitions of SCH (elevated thyroid-stimulating hormone (TSH) with normal free thyroxine 4 (FT4)) and MetS (National Cholesterol Education Program Adult Treatment Panel III (NCEP ATP III), International Diabetes Federation (IDF), or Joint Interim criteria). Data were extracted on study characteristics, prevalence rates, and metabolic associations. Quality assessment used the Newcastle-Ottawa Scale. SCH prevalence was significantly higher in MetS patients (range 8.9-37%) as compared to controls, with notable geographic and gender disparities. Key findings included: (1) Strong associations between SCH and central obesity, dyslipidemia, and hypertension; (2) Gender-specific risks, with males showing higher MetS incidence and females exhibiting higher SCH prevalence; (3) Age-related trends, with SCH prevalence increasing with age, though no elevated risk was observed in individuals ≥50 years. Longitudinal data suggested MetS may predict SCH development. This review supports a significant association between SCH and MetS, driven by shared pathways involving lipid metabolism, insulin resistance, and adiposity. Demographic variations underscore the need for tailored screening, particularly in high-risk groups. Standardized diagnostic criteria and prospective studies are warranted to clarify causality and therapeutic implications.