[Objectives] Temperature-dependent sex determination (TSD) in reptiles is increasingly threatened by climate warming. The pivotal temperature (T_piv) for the 1:1 sex ratio of reptiles shows seasonal variations and is regulated by steroid hormones. [Methods] Using Trachemys scripta elegans as a model, this study explored how maternal laying-season differences and exogenous steroid hormone treatment affected the plasticity of embryos’ critical temperature. Turtle eggs were incubated at male-producing (26.0 ℃), intermediate (28.5 ℃ and 29.0 ℃), and female-producing (31.0 ℃) temperatures, with specific exogenous steroid-hormone treatments (aldosterone, cortisol, estradiol, progesterone, dihydrotestosterone, and testosterone). Inter-group differences in female sex ratio were assessed by Chi-square tests. On the basis of temperature-dependent sex ratio data, binary logistic regression models were adopted to fit the temperature-sex response curves, from which the T_piv and the transitional range of temperatures (T_RT, producing 25% ~ 75% females) were estimated. [Results] (1) Late-season embryos had higher heat sensitivity, with T_piv 0.62 ℃ lower and T_RT 44% narrower than early-season embryos (Fig. 2). (2) Exogenous estradiol and progesterone had strong feminizing effects (Table 2), inducing 93.3% ~ 96.7% female offspring even at the male-producing temperature (26.0 ℃). (3) Exogenous cortisol and aldosterone finely regulated T_piv thresholds (max ΔT_piv = +0.30 ℃) and T_RT widths (0.06 ~ 0.90 ℃) (Fig. 3). (4) Crucially, exogenous testosterone showed significant feminizing effects at intermediate temperatures (28.5 ℃ and 29.0 ℃, Table 2). [Conclusion] This study shows that maternal seasonal laying time and embryo responses to exogenous hormone signals work together, dynamically adjusting heat-sensitivity thresholds (T_piv and T_RT) to significantly change embryo sex-determination patterns.