Cigarette smoke contains numerous bioactive compounds which may interfere with thyroid function and change the secretion of thyroid hormones. This systematic review and meta-analysis aimed to evaluate the association between smoking and thyroid hormone levels (TSH, T4, T3) by synthesizing evidence from case-control studies.

A comprehensive search of PubMed, Scopus, Web of Science, and ProQuest was conducted up to December 10, 2025. Nineteen case-control studies that measured thyroid-stimulating hormone (TSH), thyroxine (T4), and triiodothyronine (T3) were analyzed. Data extraction, risk of bias assessment, and statistical analyses (random-effects model, subgroup analyses by smoking status and gender) were performed. Heterogeneity and publication bias were assessed via I ² statistics and funnel plots. The meta-analysis included 64,060 participants (29,253 smokers, 34,807 controls).

Smoking was associated with significantly reduced TSH levels ( P < 0.001) and elevated T4 levels ( P = 0.044) while T3 levels remained unchanged ( P = 0.176). Subgroup analyses revealed stronger effects in current smokers (reduced TSH, elevated T4/T3; P ≤ 0.020) compared to former smokers, suggesting partial reversibility post-cessation. Gender differences were noted in TSH elevation ( P = 0.001) but not in T4/T3 levels. High heterogeneity (I ² > 75%) indicated confounding factors.

Smoking disrupts thyroid homeostasis, lowering TSH and raising T4 levels, with effects more pronounced in current smokers. Clinicians should consider smoking history when assessing thyroid function. The findings underscore the importance of smoking cessation for thyroid health.