Metabolic syndrome and incident thyroid cancer risk in adults: A systematic review and meta-analysis.

The global incidence of thyroid cancer (TC) has increased, with metabolic syndrome (MetS) identified as a potential modifiable risk factor. However, epidemiological evidence regarding this association remains inconsistent. We conducted a meta-analysis to quantify the relationship between MetS and the risk of incident TC in adult populations. We searched PubMed, Embase, and Web of Science for longitudinal observational studies assessing MetS in relation to incident TC. Risk ratios (RRs) with 95% confidence intervals (CIs) were pooled using random-effects models to account for heterogeneity. Our analysis included six cohort studies comprising seven independent datasets and a total of 15,730,432 participants, among whom 83,351 incident TC cases were identified during follow-up periods ranging from 2.7 to 12.0 years. The results indicated that MetS was associated with a significantly increased risk of TC (RR = 1.26, 95% CI 1.10-1.46; p < 0.001), exhibiting substantial heterogeneity (I² = 91%). This association was consistent across prospective and retrospective cohorts, varying sample sizes, follow-up durations, and different strata of TC incidence (all p for subgroup differences > 0.05).  Moreover, sex-specific analyses revealed comparable associations in both men and women. Studies with higher methodological quality (Newcastle-Ottawa Scale score = 9) demonstrated a diminished yet still significant association compared to those with scores of 7-8 (RR = 1.15 vs. 1.80; p for subgroup difference < 0.001). In conclusion, MetS is associated with a moderately increased risk of TC in adults. However, due to the observational nature of the included studies and the potential for residual confounding, these findings should be interpreted as an association rather than definitive evidence of a causal relationship.