Selenium (Se) is physiologically essential for thyroid function. However, epidemiological studies on the association between Se status and thyroid function are limited and the results are inconsistent. Therefore, we explored this association in an elderly Chinese population sample. Participants in the cross-sectional study were people aged 65 years or older who provided fingernail and whole blood samples. Hyperthyroidism and hypothyroidism were defined by serum thyroid hormones concentrations, including thyroid stimulating hormone (TSH), total triiodothyronine (TT3), total thyroxine (TT4), free thyroxine (FT3), and free thyrotropin (FT4). Significant positive association was observed between whole blood and fingernail Se concentrations (r = 0.672, P <
0.002). Compared with the lowest Se quartile (Q1), the other fingernail Se quartile groups had lower TSH, higher FT3 and FT4 levels, and Q2 had higher TT3 levels after adjusting for covariates
the other whole blood Se quartile groups had lower TSH levels, Q2 had higher FT3, FT4 and TT3 levels, Q3 had higher FT3 levels, and Q4 had higher FT4 levels after adjusting for covariates. Compared with Q1, the adjusted odds ratios (OR) and 95% confidence intervals (95%CIs) of hypothyroidism for Q4 of whole blood Se was 0.141 (0.029,0.675), and the adjusted OR (95%CIs) of hyperthyroidism for Q2 and Q3 of fingernail Se were 4.121 (1.233,13.733) and 3.614 (1.095,11.926). Higher Se levels were significantly associated with lower TSH levels and higher levels of TT3, FT3 and FT4. Meanwhile, higher Se levels were associated with lower risk of hypothyroidism and higher risk of hyperthyroidism.