Effects of Dietary Wild Thyme Alone or Combined with Probiotic or Prebiotic Supplementation on Growth Performance, Immune Response, Oxidative Status, and Intestinal Health in Broiler Chickens

(1) Background: The global transition away from antibiotic growth promoters (AGPs) requires effective, sustainable alternatives to maintain broiler performance and intestinal homeostasis. This study evaluated the effects of dietary wild thyme powder, administered alone or combined with a prebiotic or probiotic, as potential AGP alternatives on growth performance, serum oxidative status, immune response, cecal microbiota composition, short-chain fatty acid (SCFA) production, ileal histomorphometry, and intestinal barrier integrity in broilers. (2) Methods: Five hundred forty-six male Ross 308 chicks were assigned to seven treatments (n = 13 replicates/group) for 28 d: T1 (basal diet); T2 (0.5 g/kg antibiotic Neoxyval); T3 (1.0 g/kg thyme); T4 (0.75 g/kg prebiotic); T5 (0.50 g/kg probiotic); T6 (prebiotic + thyme); and T7 (probiotic + thyme). Growth performance was measured, and one bird from each replicate was randomly sampled at 28 d for multi-endpoint biological analyses. (3) Results: Birds that received T3 to T7 had increased final body weight and weight gain. Cumulative feed conversion ratio was improved in the T3, T5, and T7 groups, with T7 displaying numerical superiority over individual additive groups without reaching statistical synergy. All-natural supplements (T3, T5, T6, T7) lowered serum malondialdehyde and enhanced interferon-γ. Birds receiving T6 showed increased tumor necrosis factor-α, whereas birds receiving T3 and T4 showed reduced levels. Probiotic-inclusive diets (T5 and T7) significantly increased cecal propionic acid, total SCFAs, and ileal villus surface area. Natural additive treatments broadly promoted cecal eubiosis by increasing Lactobacillus spp. while decreasing Clostridium spp. Crucially, combining thyme with prebiotics (T6) or probiotics (T7) synergistically upregulated ileal tissue occludin concentrations. (4) In conclusion, under the conditions of this trial, dietary supplementation with wild thyme powder, a MOS/β-glucan prebiotic, and a Bacillus licheniformis probiotic, particularly the thyme + probiotic combination, was an effective regimen for supporting growth performance, cecal eubiosis, and ileal barrier integrity in healthy broilers. However, these findings are limited to a single dose level (1.0 g/kg thyme), a single plant source, and non-challenged conditions; further research incorporating dose–response evaluations, disease-challenge models, and diverse phytogenic origins is necessary to confirm broader field applicability.