BACKGROUND: Although recent studies indicate a high prevalence of subclinical hypothyroidism (SCH) in women with polycystic ovary syndrome (PCOS), the reported prevalence rates vary widely. Therefore, we conducted this study to estimate the pooled prevalence of SCH among women with PCOS. Additionally, emerging evidence suggests that SCH may negatively impact insulin resistance in PCOS. Thus, we examined its effect on insulin resistance indices as our secondary objective. METHODS: We searched PubMed, Web of Science, Scopus, and Embase from their inception to February 25, 2024. Observational studies reporting the prevalence of SCH among women with PCOS were included. Joanna Briggs Institute's (JBI) critical appraisal checklist for prevalence studies was adopted for the risk of bias assessment. The random-effects model was employed to estimate the pooled prevalence with its 95% confidence intervals (CI). The weighted mean difference (WMD) was used to compare the insulin resistance indices between PCOS patients with and without SCH. RESULTS: Twenty-nine studies comprising 5765 women with PCOS were included. The meta-analysis demonstrated that 19.7% (95% CI: 16.1%
23.5%) of women with PCOS have SCH. PCOS patients with SCH had significantly higher HOMA-IR (WMD = 0.78, 95% CI: 0.34
1.22) and fasting insulin (WMD = 2.38, 95% CI: 0.34
4.42) levels than those without SCH. Differences in fasting plasma glucose and 2-hour postprandial glucose did not reach statistical significance. CONCLUSION: This systematic review and meta-analysis found that approximately 20% of women with PCOS have SCH. This underscores the need for regular thyroid function testing in these patients. The prevalence of SCH is influenced by the TSH cut-off used for diagnosis, highlighting the need for establishing a standardized TSH cut-off value. Furthermore, SCH significantly elevates the HOMA-IR index and fasting insulin levels, highlighting its potential impact on insulin resistance. Whether these metabolic changes are clinically important and put these individuals at higher risk of developing type 2 diabetes mellitus and cardiovascular disease requires further investigation. SYSTEMATIC REVIEW REGISTRATION NUMBER IN PROSPERO: CRD42024510798. CLINICAL TRIAL NUMBER: Not applicable.