Background <p>The association between season and thyroid function parameters has been extensively studied, but the non-linear and lagged effects of extremely low ambient temperature on thyroid function parameters remain unknown.</p> Objective <p>We sought to examine non-linear and lagged patterns in how extremely low ambient temperature is associated with thyroid function parameters.</p> Methods <p>We conducted a panel study based on the data of five thyroid function parameters from the 949th Hospital in Altay City, China, from 2020 to 2023. Mixed-effect modeling with distributed lagged non-linear models (DLNMs) was applied to analyze the data.</p> Results <p>Thyroid function parameters from 1237 individuals were collected during the study period. Compared with the benchmark temperature (8.5°C), free triiodothyronine (FT3) and free thyroxine (FT4) levels increased significantly by 2.01 (95% CI, 0.82, 3.19 pg/mL), and 0.80 (95% CI, 0.24, 1.36 pg/mL) in − 20°C, respectively. While thyroid-stimulating hormone (TSH) level increased by 0.91 μU/mL compared with the benchmark temperature (8.5°C), albeit without a significant difference (95% CI, − 0.50, 2.32). When temperature increased from 20°C to 29°C, total thyroxine (TT4) significantly increased and FT4 significantly decreased. Conversely, the changes in total triiodothyronine (TT3) levels under different temperatures were more subtle. Further subgroup analyses showed that under extremely low ambient temperatures, women had greater increases compared to men in TT4 level and individuals with thyroid disorders had greater increases in TT4, FT4 and FT3 levels than those without thyroid disorders.</p> Conclusions <p>This study provides new epidemiological evidence that extremely low ambient temperatures may affect thyroid function. Therefore, timely protective measures during cold weather may be helpful to maintain stable thyroid function and to reduce the risk of hypothermia and frostbite, especially in people with thyroid disorders.</p>