Background The escalating challenges of fine particulate matter (PM_2.5) pollution and global warming pose significant threats to human health. Previous studies have demonstrated that prenatal exposure to PM_2.5 and heat is associated with restricted fetal and offspring growth. However, the combined effects of co-exposure to PM_2.5 and heat remain poorly understood, and the underlying biological mechanisms have yet to be elucidated. Objective To investigate the independent and combined effects of maternal exposure to PM_2.5 and heat on offspring growth and thyroid hormone levels in rats. Methods Eight-week-old SPF Wistar rats were used. Females and males were mated at a 2:1 ratio. After confirming pregnancy, 36 pregnant rats were randomly assigned to four groups: combined PM_2.5 and heat exposure, PM_2.5 exposure, heat exposure, and control. PM_2.5 exposure was administered via intratracheal instillation (0.3 mL, 2 mg·mL⁻¹) daily. Heat exposure was conducted in a programmable constant temperature and humidity chamber maintained at (38.0±1.0)℃ for 1 h daily. Exposures were administered from gestational days (GD) 7 to 13. Offspring were raised for 42 postnatal days (PND), with body weight measured twice weekly at fixed time points. On PND42, blood was collected to separate plasma for the measurement of free triiodothyronine (FT₃) and free thyroxine (FT₄) levels using enzyme-linked immunosorbent assay. Linear mixed-effects models were used to examine the interactive effects of PM_2.5 and heat exposure on offspring body weight and weight gain. One-way analysis of variance was conducted to compare the differences in offspring body weight at each time point and thyroid hormone levels among groups. Correlations between thyroid hormone levels and weight changes were assessed using Spearman's rank correlation analysis. Results Both prenatal PM_2.5 exposure alone and its combination with heat exposure were associated with higher offspring body weight at specific postnatal time points. Offspring in the PM_2.5 exposure group had higher body weight than controls on PND7 and PND21, while those in the combined exposure group had higher body weight on PND7 and PND21-35 (P<0.05), and this effect was stronger in female offspring. The heat exposure group showed significantly lower birth weight than other groups (P<0.05), but this difference gradually disappeared during development. Plasma FT₃ levels were higher in the PM_2.5 exposure group than in the heat exposure and the control groups (P<0.05), but this difference was statistically significant only in female offspring. Plasma FT3 levels were positively correlated with body weight from PND7 to PND42, with correlation coefficients of 0.316–0.539 in all offspring (P<0.05) and 0.419–0.706 in females (P<0.05), while no significant correlation was observed in males. Conclusion Prenatal exposure to PM_2.5 alone or a combination of PM_2.5 and heat may affect postnatal body weight gain in offspring in a time-dependent manner, with more pronounced effects observed in female offspring. Heat exposure alone leads to reduced early-stage weight followed by a trend toward catch-up growth. The findings suggest a specific role of thyroid hormones in the effects of prenatal PM_2.5 and heat exposure on offspring growth, a relationship that warrants further investigation.