Objective To explore the relationship between thyroid hormone and colorectal adenomatous polyps (CAP) in middle-aged and elderly people. Methods We surveyed 3 042 middle-aged employees and retirees aged 45 years and over at a petrochemical enterprise of Ningbo. Their colonoscopic findings, body mass index, waist circumference and thyroid function were analyzed. Results Age and serum free triiodothyronine (FT3) of females with CAP were higher than those of the corresponding non-CAP group, so were age and waist circumference of males with CAP (P<0.05). Furthermore, these differences were statistically significant. The detection rate of CAP was significantly higher in males than in females (χ2=49.418, P<0.01). The detection rate of CAP was higher in females with higher levels of FT3. Multivariate unconditional logistic regression analyses showed the effects of thyroid hormone on the CAP differed by gender. Age and FT3 (OR=1.60, 95%CI: 1.12-2.28, P=0.01) were correlated with CAP in females. However, only age and waist circumference were correlated with CAP in males. Conclusion Elevated FT3 may be a risk factor of CAP in elderly females. The detection rate of CAP may not be affected by FT3 levels in males. Key words: Colorectal neoplasms; Colorectal adenomatous Polyps; Thyroid function