Research evidence indicates that dietary supplementation with thyme (Thymus vulgaris) exerts multiple beneficial effects in poultry production. These effects, summarized in Figure 1 and Figure 2, include improvements in growth performance, carcass characteristics, blood biochemical parameters, immune response, gut health, and overall economic efficiency.

Table 1 provides a thorough summary of experimental research assessing the impact of supplementing poultry with thyme-based additives on growth performance. The growth performance of broiler chickens can be positively impacted by dietary supplementation with thyme, either as leaf powder or essential oil, according to numerous studies. However, responses seem to be dose-dependent and depend on the type of supplementation. According to Deeb et al. (2024)(Deeb et al., 2024), supplementing with thyme leaf powder (TLP) at concentrations between 1 and 3 g/kg increased body weight and daily body weight gain (DBWG) at various growth stages. Birds receiving 3 g TLP/kg exhibited the highest body weight at 15 days of age, while inclusion levels of 1 and 2 g/kg resulted in superior final body weight and DBWG over the entire rearing period (days 1-38). Improvements in feed conversion ratio (FCR) were particularly evident at inclusion levels of 2 and 3 g TLP/kg, indicating enhanced feed efficiency.

At higher inclusion levels, thyme powder has also been shown to consistently promote growth. In broilers fed diets supplemented with 5 g/kg thyme powder, Hassan and Awad (2017)(Hassan & Awad, 2017) found significant increases in body weight, body weight gain, and feed intake. Similarly, despite no discernible changes in feed intake, Abdel-Wareth et al. (2012)(Abdel-Wareth et al., 2012) showed that supplementing a diet with 15 or 20 g/kg of thyme significantly increased body weight gain and improved FCR when compared to a basal control diet. The observed improvements were ascribed to the bioactive qualities of thyme because all experimental diets were isoenergetic and isonitrogenous.

On the other hand, at lower inclusion levels, some studies have found inconsistent or limited effects. The final body weight, body weight gain, FCR, and protein efficiency ratio were all marginally improved by dietary thyme leaf powder supplementation at 0.5%–1.5%, according to Abdel-Ghaney et al. (2017)(Abdel-Ghaney et al., 2017). However, these improvements were not statistically significant (p>0.05). Similarly, Ocak et al. (2008)(Ocak et al., 2008) found that in Ross 308 broilers aged 7 to 35 days, supplemented with 0.2% thyme leaf (equivalent to 70 mg thymol/kg diet) had no discernible effect on body weight gain, feed intake, or FCR. Notably, these differences among studies are likely due to the potency of the thyme used, specifically the concentration of active compounds like thymol, and the timing of when the supplement was introduced to the chicks’ diet. Scientifically, if the dosage or the bird’s age at the start of the study is not optimal, the thyme may fail to reach the biological threshold necessary to produce a measurable impact.

Thyme essential oil (TEO) has generally produced more consistent responses at lower inclusion rates compared with powder forms because it provides a highly concentrated, standardized dose of bioactive phenols like thymol and carvacrol, whereas powder forms are subject to the natural variability of plant fiber, soil quality, and harvest conditions. Because the oil isolates these active metabolites from the inert plant matrix, it offers superior bioavailability and more predictable physiological effects. Furthermore, TEO can be easily emulsified or encapsulated to ensure uniform distribution and protection during digestion, allowing for precise, potent supplementation that avoids the bulk and instability associated with raw botanical powders. Adam et al. (2020)(Adam et al., 2020) reported that supplementation with thyme oil at 100 or 200 mg/kg significantly improved final body weight, body weight gain, and FCR in Arbor Acres broilers, with the greatest response observed at 200 mg/kg, as at this inclusion level, feed intake was notably decreased, indicating improved growth efficiency. Comparable results were reported by Almremdhy and Al-Khafaji (2020)(Almremdhy & Al-Khafaji, 2020), who administered thyme oil via drinking water (0.3 mL/L) and observed significant (p<0.05) improvements in body weight, weight gain, and FCR, accompanied by reduced feed intake.

The phenolic components of thyme, thymol, and carvacrol, which have potent antimicrobial and antioxidant properties that may improve digestion, nutrient absorption, and gut microbial balance, have also been linked to the growth-promoting benefits of thyme(Cross et al., 2007); (Hoffman-Pennesi & Wu, 2010). Thyme powder or essential oils have been shown to improve broiler growth performance in several previous studies(Toghyani et al., 2010);(Foroughi et al., 2011);(El-Sabrout et al., 2023);(Soliman et al., 2024).

Under environmental stress, thyme supplementation appears particularly effective. Attia et al. (2017)(Attia et al., 2017) demonstrated that dietary thyme oil supplementation at 1.0-2.0 g/kg significantly improved FCR in heat-stressed Ross 308 broilers (p<0.01). Similarly, Khafar et al. (2019)(Khafar et al., 2019) reported that thyme essential oil at 150 or 200 mg/kg mitigated the negative effects of heat stress by improving body weight gain and feed efficiency while reducing FCR. TEO mitigates heat stress by acting through a synergistic multi-pathway mechanism that prioritizes metabolic efficiency and cellular protection. Primarily, its phenolic components, thymol and carvacrol, neutralize reactive oxygen species (ROS) and boost endogenous antioxidant enzymes, preventing the oxidative damage to tissues typically caused by high temperatures. Simultaneously, TEO stimulates the secretion of digestive enzymes and improves intestinal morphology, specifically increasing villus height, ensuring that nutrient absorption remains high even when heat-induced blood flow is diverted from the gut. By suppressing systemic inflammation and downregulating heat shock proteins, TEO reduces the metabolic energy “tax” of the stress response, allowing the animal to redirect its physiological resources toward weight gain and improved feed conversion ratios(Büyükkılıç Beyzi et al., 2020);(Mahasneh et al., 2024).

Comparative studies have also shown that thyme (powder) can perform similarly to, or better than, conventional antibiotic growth promoters. Hassan et al. (2024)(Hassan et al., 2024) reported that supplementation with thyme (1.0% of diet), ginger (1.0%), or their nanoparticles (0.10%) forms significantly improved body weight gain and FCR compared with a diet containing bacitracin methylene disalicylate. In addition, Hesabi Nameghi et al. (2019)(Hesabi Nameghi et al., 2019) observed that thyme–peppermint–eucalyptus essential oil blend administered via drinking water (150 ppm) significantly improved production index, body weight gain, and feed efficiency in broilers. Likewise, Nameghi et al. (2022)(Nameghi et al., 2022) reported that a combination of thyme and rosemary powders (0.375% each) improved average daily gain and FCR compared with a positive control diet.

In other poultry species, responses to thyme supplementation have been variable. In Japanese quail, Maulod et al. (2022)(Maulod et al., 2022) reported that dietary thyme powder at 0.5-1.0% significantly increased live body weight and body weight gain without affecting feed intake. Khaksar et al. (2012)(Khaksar et al., 2012), similarly reported increased live body weight following supplementation with 1 g/kg thyme essential oil. Conversely, Kheiri et al. (2018)(Kheiri et al., 2018) found no significant effects of thyme supplementation at 2 g/kg on growth performance or feed intake in Japanese quail. Eidrisha et al. (2022)(Eidrisha et al., 2022) further highlighted species-and dose-dependent variability, reporting that thyme supplementation (5 or 10 g/kg) did not significantly enhance growth performance compared with other herbal additives.

Thyme supplementation enhances poultry reproductive performance and egg quality primarily by leveraging its potent antioxidant properties to protect highly metabolic reproductive tissues and lipid-rich egg components (Table 1). The transfer of bioactive phenols, such as thymol and carvacrol, into the egg yolk reduces lipid peroxidation and malondialdehyde (MDA) levels, which maintains the Haugh unit and extends egg freshness. Furthermore, thyme supports shell integrity by improving mineral metabolism and protecting the shell gland’s mucosal health, while simultaneously optimizing follicular development in the ovaries and sperm viability in males. By mitigating oxidative stress and inhibiting cholesterol-synthesis enzymes, thyme ensures a more persistent laying rate, improved fertility, and the production of nutritionally superior eggs with lower cholesterol and higher oxidative stability(Yalçın et al., 2020);(Hassanpour et al., 2025). Evidence regarding the effects of thyme supplementation on reproductive performance and egg quality in poultry is limited but promising. Male Gimmizah chickens given thyme leaf aqueous extract (TLAE) at a dosage of 4 mL/10 L of drinking water showed notable improvements in semen quality parameters, such as sperm motility, morphological characteristics, velocity, and kinetic traits, according to El-Hadad et al. (2024)(El-Hadad et al., 2024). These results imply that thyme bioactive compounds have a beneficial effect on male reproductive function. Although there were no discernible effects on fertility or hatchability rates, dietary administration of TLAE at 4 mL/10 L significantly increased egg yield, hatched chick weight, and the chick–ovo index in laying hens. These findings suggest that supplementing with thyme may improve early chick performance and reproductive output without negatively impacting reproductive efficiency. Furthermore, a recent study by Ebrahimi et al. (2025)(Ebrahimi et al., 2025) reported that adding 0.5% thyme powder to laying hen diet can improve performance and the number of large white follicles, as well as decrease the level of triglycerides and cholesterol in egg yolk and blood serum.

With respect to egg quality, supplementation with TLAE and TEO resulted in marked improvements in several physical and internal egg quality traits; specifically, egg weight, length, width, Haugh unit, and shape index were significantly increased compared with those of the control group(El-Hadad et al., 2024);(Song et al., 2026). Moreover, improvements were observed in albumen weight, height, and index, as well as yolk height and yolk index, indicating an overall enhancement of egg structural quality and freshness. In the same manner, recent findings by Baser et al. (2025)(Baser et al., 2025) demonstrated that supplementing Chukar partridge diets with 50 mg/kg of TEO significantly boosts egg production and mass without compromising performance or mineral balance. These improvements are likely driven by active compounds like thymol and carvacrol, which facilitate better nutrient utilization by enhancing the structural integrity of the intestinal lining, increasing the height of the villi to provide a larger surface area for nutrient uptake, while simultaneously suppressing pathogenic microflora that would otherwise compete for energy. Additionally, they improve the net “bioavailability” of the diet; these compounds ensure that a higher proportion of ingested protein, minerals, and energy is partitioned away from metabolic waste and toward the physiologically demanding process of follicular development and egg formation. Notably, higher doses did not provide additional benefits; instead, they negatively impacted metrics such as eggshell thickness and yolk index, potentially due to impaired nutrient absorption. Consequently, 50 mg/kg appears to be the optimal dosage for enhancing partridge productivity.

Data are relatively scarce regarding the economic gain of thyme supplementation in poultry production (Table 1). However, in spite of this, the study of Hassan and Awad (2017)(Hassan & Awad, 2017) has resulted useful information in terms of cost–benefit effect of thyme inclusion on broiler diet. The economic analysis performed by the mentioned authors showed that a high level of thyme powder inclusion (8 g/kg diet) was not economically viable since it resulted in higher costs with feed and net revenue per kilogram of live weight was the lowest. In contrast, broilers fed thyme-supplemented diets with moderate concentrations of thyme (2 or 5 g/kg) were equal to or better than the control group in terms of economic performance; thus, supplementation at a level of 5 g/kg represented the most favorable cost/performance with regard to growth and feed costs as well as overall profitability, emphasizing that experimental optimization for inclusion levels would ensure high economy when using thyme as phytogenic feed additive.

Dietary supplementation with thyme, provided as dried leaves, powder, or essential oil, has been reported to influence carcass characteristics and meat quality in poultry (Table 2). The scale and nature of the changes appear to vary by the dose used and the type of poultry. In broiler chickens, Deeb et al. (2024)(Deeb et al., 2024) have indicated that TLP (1-3 g/kg diet) has had minimal influence on most carcass characteristics. There were no dietary treatment differences in the dressing percentage, abdominal fat, and heart gizzard, and distributions of spleen; no linear or quadratic changes were noted as TLP concentration increased. However, liver weight expressed as a percentage of body weight was significantly reduced in birds fed 3 g/kg TLP compared with the control group, suggesting a potential modulatory effect of thyme on hepatic metabolism or metabolic load.

Contrasting results have been reported in other studies. Khaksar et al. (2012)(Khaksar et al., 2012) demonstrated that supplementation of Japanese quail diets with 1 g/kg TEO significantly increased carcass and breast yields compared with those of the control group. Similarly, Lee et al. (2003a)(Lee et al., 2003) observed that thymol supplementation increased liver weight in broilers at 21 days of age, although this effect was not evident at 40 days. In contrast, Ocak et al. (2008)(Ocak et al., 2008) reported no significant effects on carcass weight or dressing percentage in Ross 308 broilers fed diets containing 0.2% thyme leaves up to 42 days of age. Nevertheless, both herbs significantly increased abdominal fat pad weight, indicating a possible trade-off between growth-promoting effects and carcass fat deposition.

Adam et al. (2020)(Adam et al., 2020) further reported that while dressing percentage and relative organ weights were largely unaffected by thyme supplementation, the inclusion of thyme powder (2.5-5 g/kg) or thyme oil (100-200 mg/kg) consistently reduced abdominal fat content in broilers. The lowest abdominal fat values were observed in birds receiving 200 mg/kg thyme oil, suggesting a dose-dependent lipolytic or metabolic regulatory effect. Conversely, no significant changes in carcass characteristics were detected in quail fed diets supplemented with thyme powders at 5 or 10 g/kg(Eidrisha et al., 2022).

Hassan et al. (2024)(Hassan et al., 2024) reported that dietary supplementation with thyme at 1.0%, as well as thyme and ginger nanoparticle forms at 0.10%, did not significantly affect carcass yield or most organ weights in broilers. However, an increase in the relative weight of the bursa of Fabricius and a reduction in abdominal fat were observed compared with the control group. In addition to carcass traits, thyme, ginger, and their nanoforms positively influenced meat quality attributes, including color, water-holding capacity, and sensory characteristics, with effects comparable to those observed in antibiotic-supplemented birds. Similarly, supplementation with a thyme–peppermint–eucalyptus essential oil blend (100-200 ppm) significantly increased carcass yield and thigh muscle proportion in Ross 308 broilers(Hesabi Nameghi et al., 2019). Khaksar et al. (2012)(Khaksar et al., 2012) found that TEO supplementation significantly increased the percentages of carcass and breast in Japanese quail, but had no effect on the relative weights of the thighs, drumsticks, spleen, liver, heart, or abdominal fat. These results are in line with earlier research showing that thyme extract powder or essential oil typically has no effect on the weights of the pancreas, spleen, liver, or heart(Hernández et al., 2004);(Sarica et al., 2005). Similarly, Kheiri et al. (2018)(Kheiri et al., 2018) found that dietary thyme (2 g/kg) had no effect on organ weights or carcass yield in Japanese quail, with yields of breast and leg meat that were similar to those of control and antibiotic-treated birds.

More recently, Maulod et al. (2022)(Maulod et al., 2022) reported that the inclusion of 0.5%-1% thyme powder in feed increased the dressing percentage in Japanese quail females (with no changes in males) and that thyme-supplemented quail had better breast, wing, leg, and back proportions than control quail.

Dietary supplementation with thyme, provided as leaf powder, aqueous extract, or essential oil, has been consistently associated with modifications in blood biochemical and hematological parameters in poultry, reflecting improvements in metabolic status, liver and kidney function, immune competence, and lipid metabolism (Table 3).

Deeb et al. (2024)(Deeb et al., 2024) reported that TLP (1-3 g/kg diet) induced linear responses in serum total protein, albumin, the albumin/globulin (A/G) ratio, alanine aminotransferases (ALT), and creatinine levels in broiler chickens. Notably, creatinine concentrations were significantly lower in birds fed 2 or 3 g/kg TLP compared with the control and 1 g/kg groups, suggesting improved renal function. In contrast, serum globulin and aspartate aminotransferase (AST) levels were not affected by TLP supplementation. Similarly, Abdel-Ghaney et al. (2017)(Abdel-Ghaney et al., 2017) observed increased serum total protein and globulin concentrations in broilers supplemented with thyme at 5 or 15 g/kg during the later stages of growth, whereas albumin, creatinine, and ALT levels remained unchanged. These findings suggest that thyme may enhance protein metabolism without exerting adverse effects on liver or kidney function.

Several studies have demonstrated that thyme supplementation positively modulates hematological/biochemical indices associated with immune competence(El-Ghousein & Al-Beitawi, 2009);(Khatun et al., 2025);(Yasin et al., 2025). In addition, thyme supplementation in broiler diets (powder or oil) has a substantial beneficial effect on hematological responses, as the bioactive phenols (thymol and carvacrol) exert a protective effect on red blood cell membranes by neutralizing free radicals, which reduces oxidative hemolysis and maintains higher hemoglobin levels and packed cell volume. Furthermore, thyme acts as an immunomodulator, often leading to a more favorable white blood cell profile. By lowering systemic inflammation and protecting hematopoietic tissues, thyme ensures a more robust circulatory system capable of efficient oxygen transport and superior immune defense. Total white blood cells (WBC), lymphocyte, and heterophil counts were elevated in birds fed 2.5 or 5 g of thyme powder/kg diet and those receiving 100 or 200 mg thyme oil/kg diet(Adam et al., 2020). The heterophil: lymphocyte ratio was similarly beneficial, indicating that there was an improvement in the immune competence of the broilers and decreased physiological stress. Adam et al. (2020)(Adam et al., 2020) reported increased total white blood cell counts, lymphocyte percentages, and heterophil numbers in broilers fed thyme powder (2.5-5 g/kg) or thyme oil (100-200 mg/kg). The heterophil-to-lymphocyte ratio was favorably reduced, indicating lower physiological stress and enhanced immune responsiveness. Consistent immunostimulatory effects were also reported by Hassan and Awad (2017)(Hassan & Awad, 2017), who observed increased white blood cell and lymphocyte counts in broilers fed 2 or 5 g/kg thyme, supporting the role of thyme as a natural immune-modulating feed additive.

Recent studies show that thyme supplementation has positive effects on lipid metabolism. Adam et al. (2020)(Adam et al., 2020) indicated that thyme powder/oil decreases serum triglycerides, total cholesterol, and very-low-density lipoprotein (VLDL) with concomitant increases in levels of high-density lipoprotein (HDL), especially in the case of broiler chickens that were provided with 200 mg/kg thyme oil. Aldik et al. (2020)(Aldik et al., 2020) researched the hypolipidemic effects of thyme and reported that the supplementation of 4-6 g/kg thyme powder to broilers increased HDL and globulin concentrations and decreased serum glucose, total cholesterol, triglycerides, and low-density lipoprotein (LDL). Comparable improvements in lipid profiles were reported in broilers receiving thyme–peppermint–eucalyptus essential oil blends(Hesabi Nameghi et al., 2019) and in birds fed a combination of thyme and rosemary powders(Nameghi et al., 2022), further supporting the lipid-lowering potential of thyme-based phytogenic additives.

The bioactive phenolic monoterpenes thymol and carvacrol are the primary mediators of the physiological activities of Thymus vulgaris on the hematological and biochemical profiles of birds. By inhibiting the activity of the rate-limiting enzyme β-hydroxy-β-methylglutaryl-coenzyme A (HMG-CoA) reductase in the hepatic biosynthesis of cholesterol, and concomitantly enhancing the activity of lipoprotein lipase (LPL) to facilitate the hydrolysis of triglycerides and VLDL, thymol and carvacrol produce a hypolipidemic effect. Thyme also possesses immunomodulatory activity, enhancing the proliferation of lymphocytes and the serum globulin levels, indicating an improved humoral immunity. The high antioxidant activity of the flavonoids in thyme reduces lipid peroxidation, stabilising the cell membranes and ensuring the maximum efficiency of the liver’s metabolic activities, as evidenced by the decrease in serum glucose levels, indicating improved insulin sensitivity and glucose metabolism(Nagoor Meeran et al., 2017);(Saied et al., 2025).

Furthermore, thyme supplementation has been demonstrated to alleviate metabolic and oxidative disturbances under conditions of heat stress. Khafar et al. (2019)(Khafar et al., 2019) found that TEO (150–200 mg/kg diet) restored levels of corticosterone, malondialdehyde, triglycerides, and cholesterol to normal in heat-stressed broilers. This shows that the antioxidant capacity and metabolic stability of the birds improved. Attia et al. (2017)(Attia et al., 2017) also found that broilers given thyme oil (1.5–2.0 g/kg) had higher levels of total protein and globulin in their blood and lower levels of AST activity. This shows that thyme oil protects the liver and boosts the immune system. El-Hadad et al. (2024)(El-Hadad et al., 2024) found that adding TLAE (4 ml/10 L drinking water) to Gimmizah chickens’ diets raised their hemoglobin, hematocrit, glucose, creatinine, and cholesterol levels while lowering their AST, ALT, and malondialdehyde levels.

In Japanese quail, responses to thyme supplementation appear more variable. Triglycerides and serum protein fractions were not significantly affected by thyme powder, according to Eidrisha et al. (2022)(Eidrisha et al., 2022). On the other hand, Khaksar et al. (2012)(Khaksar et al., 2012) and Maulod et al. (2022)(Maulod et al., 2022) showed that quail supplemented with thyme essential oil or powder had significantly lower serum triglycerides, total cholesterol, LDL, and glucose levels, along with higher HDL concentrations. In a similar vein, Kheiri et al. (2018)(Kheiri et al., 2018) found that supplementing with thyme raised HDL levels but had little effect on total cholesterol and protein metabolism. Thymol and carvacrol, two of thyme’s bioactive phenolic compounds, are mainly responsible for its hypolipidemic effects. According to Lee et al. (2003a, b)(Lee et al., 2003);(Lee et al., 2003), these substances have been demonstrated to inhibit hepatic 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, a crucial enzyme involved in the biosynthesis of cholesterol.

Recent evidence from Saied et al. (2025)(Saied et al., 2025) indicates that thyme oil supplementation effectively modulates lipid profiles, leading to a reduction in serum cholesterol. Likewise, a recent meta-analysis by Hassanpour et al. (2025)(Hassanpour et al., 2025) demonstrated that thyme-based additives consistently exert hypocholesterolemic effects across various poultry trials. Nevertheless, the magnitude of these responses may vary across breeds, sexes, ages, environmental conditions, and the synergistic effects of other dietary ingredients.

Dietary supplementation with thyme has been widely reported to enhance both immune competence and antioxidant defenses in poultry, largely due to the presence of phenolic compounds such as thymol and carvacrol. A synthesis of the main immunological and antioxidant responses observed in different poultry species and experimental conditions is provided in Table 4. According to Deeb et al. (2024)(Deeb et al., 2024), serum immunoglobulin Y (IgY) and IgM concentrations were higher in broiler chickens fed TLP (1-3 g/kg diet) than in the control group, indicating a significant improvement in humoral immunity. The highest IgM levels were observed in birds fed a 2 g TLP/kg, indicating that moderate supplementation levels may be optimal for stimulating antibody-mediated immune responses.

Consistent findings were reported by Abdel-Ghaney et al. (2017)(Abdel-Ghaney et al., 2017), who observed significant increases in serum IgG, IgM, interferon-gamma (IFN-γ), and interleukin-10 (IL-10) concentrations in broilers fed diets supplemented with 5, 10, or 15 g/kg thyme. These results suggest that thyme supplementation enhances both humoral and cell-mediated immunity. Similarly, Hassan and Awad (2017)(Hassan & Awad, 2017) reported elevated IgG levels and increased nitric oxide (NO) production in broilers fed 5 g/kg thyme, reflecting stimulation of both adaptive and innate immune responses. Enhanced immune organ development and antibody titers against infectious bursal disease (IBD) were also observed in broilers receiving thyme oil supplementation (0.3 mL/L drinking water), along with increased relative weights of the bursa of Fabricius, thymus, and spleen(Almremdhy & Al-Khafaji, 2020).

Thyme supplementation has also been associated with improved vaccine-induced immune responses. Talazadeh and Mayahi (2017)(Talazadeh & Mayahi, 2017) reported enhanced antibody titers following influenza vaccination in birds receiving thyme extract, whereas Attia et al. (2017)(Attia et al., 2017) observed increased white blood cell counts and elevated antibody titers against IBD in broilers supplemented with thyme oil (1.0-2.0 g/kg) or mannanoligosaccharides. However, inconsistent findings have been reported in some studies, as Teymouri Zadeh et al. (2009)(Teymouri Zadeh et al., 2009) and Rahimi et al. (2011)(Rahimi et al., 2011) did not observe significant improvements in antibody responses following thyme supplementation. These discrepancies may be attributed to differences in thyme chemotype, dosage, administration route, vaccination protocols, or bird age. In addition to its immunomodulatory effects, thyme supplementation exerts a pronounced influence on antioxidant status. Deeb et al. (2024)(Deeb et al., 2024) reported increased superoxide dismutase (SOD) activity and reduced malondialdehyde (MDA) concentrations in broilers fed TLP, with the most favorable antioxidant profile observed at 1 g/kg supplementation. Similar antioxidant-enhancing effects were reported by Aldik et al. (2020)(Aldik et al., 2020), who observed improved globulin concentrations and albumin-to-globulin ratios in broilers supplemented with 4-6 g/kg thyme, reflecting enhanced protein synthesis and antioxidant capacity.

Abdel-Ghaney et al. (2017)(Abdel-Ghaney et al., 2017) showed that thyme supplementation dramatically raised muscle antioxidant enzymes, such as glutathione, SOD, and glutathione S-transferase, while lowering MDA levels, indicating less oxidative damage and lipid peroxidation. Similar findings have also been noted by Hesabi Nameghi et al. (2019)(Hesabi Nameghi et al., 2019), who observed enhanced humoral immunity and antioxidant status in broilers supplemented with thyme–peppermint–eucalyptus essential oil blends at up to 200 ppm. In Gimmizah chicken, El-Hadad et al. (2024)(El-Hadad et al., 2024) demonstrated that aqueous extract of thyme leaves (4 ml/10 L drinking water) effectively decreased plasma MDA concentration, while triiodothyronine (T₃), IgG, IgM, and total antioxidant capacity were increased. These results show that the aqueous extract of thyme leaves is effective in modulating endocrine, immune, and antioxidant status. The immunomodulatory and antioxidant effects of thyme are largely attributed to its phenolic constituents, which exhibit strong free radical scavenging activity, enhance endogenous antioxidant enzyme systems, and modulate cytokine production. By reducing oxidative stress and supporting immune cell function, thyme supplementation contributes to improved health resilience, particularly under challenging environmental or pathological conditions.

One of the main mechanisms by which phytogenic feed additives, such as thyme, have a beneficial impact on broiler performance and health is through their ability to modulate intestinal morphology and microbiota. A brief account of the main findings related to broiler response, including gut morphology and microbiota, following dietary supplementation with thyme is provided in Table 5. Thyme supplementation has been found to modulate broiler microbiota, inhibiting harmful bacteria while increasing beneficial ones. Aldik et al. (2020)(Aldik et al., 2020) found that broilers fed diets supplemented with 4-6 g/kg feed of thyme had lower total aerobic bacterial and coliform counts, both in the duodenum and cecum, while showing a high increase in Lactobacillus populations. This shows that thyme leaf powder, while being a selective antimicrobial agent, is beneficial to broilers, especially in creating a balanced microbiota, thus helping them utilize nutrients better. Abdel-Wareth et al. (2012)(Abdel-Wareth et al., 2012) found that there was a decline in Lactobacillus spp. counts, while total bacterial counts were unaffected, following dietary supplementation with thyme, indicating that thyme might have a selective impact on microbiota rather than being a general antimicrobial agent. Thyme’s selective antimicrobial action is primarily dictated by the lipophilic nature of thymol and carvacrol, which preferentially disrupt the phospholipid bilayers of specific bacteria. While many Gram-negative pathogens like E. coli and Salmonella are highly sensitive to this membrane destabilization, the impact on beneficial Gram-positive bacteria like Lactobacillus spp. is often dose-dependent.

Besides this microbial modulation effect, thyme supplementation has also been observed to have a positive effect on intestinal morphological characteristics that are associated with increased absorptive capacity. In this regard, it has been observed by Hesabi Nameghi et al. (2019)(Hesabi Nameghi et al., 2019) that thyme-peppermint-eucalyptus essential oil blends at a concentration of 150-200 ppm increased ileal villus height and ileal VH:CD ratio in broilers, indicating improved intestinal integrity and epithelial turn-over. Moreover, the higher inclusion level (200 ppm) reduced ileal Escherichia coli counts, while 150 ppm in-creased Lactobacillus populations at both 21 and 42 days of age. Similarly, Nameghi et al. (2022)(Nameghi et al., 2022) demonstrated that dietary inclusion of a blend containing 0.375% thyme and 0.375% rosemary significantly increased ileal villus height and VH:CD ratio, accompanied by an increased abundance of beneficial ileal microbiota. These morphological and microbial adaptations suggest improved digestive efficiency and nutrient absorption in thyme-supplemented birds.

In Japanese quail, Khaksar et al. (2012)(Khaksar et al., 2012) reported that supplementation with TEO (1 g/kg diet) significantly increased ileal Lactobacillus populations and reduced Escherichia coli counts, indicating a favorable shift in the intestinal microbial ecosystem. The antimicrobial effects were attributed to the phenolic constituents of thyme oil, particularly thymol and carvacrol, which have been shown to inhibit the growth of E. coli and Salmonella typhimurium in vitro. However, the results of the same study indicated variable results in the different experimental conditions. The efficacy of thyme essential oil in modulating the intestinal microbial ecosystem may be dependent on the diet formulation, concentration of the essential oil, health status of the flock, and the environment(Khaksar et al., 2012).

Overall, the above findings suggest that the supplementation of thyme may improve gut health by exerting its selective antimicrobial properties and improving the morphology of the intestine. This may lead to an improvement in the digestibility of nutrients and the performance of the animal. However, the conflicting findings of the above studies suggest that the inclusion levels and other factors, such as species and environment, should be considered for the application of thyme-based feed additives in poultry.

The effects of thyme supplementation on gene expression in poultry remain underexplored, though recent studies have provided some valuable insights into its potential role in regulating immune and intestinal barrier functions. A summary of the gene expression data related to thyme supplementation is presented in Table 6. Thyme supplementation has been shown to influence the expression of genes involved in the inflammatory response, with significant implications for gut health and overall immunity. In this context, Hassan and Awad (2017)(Hassan & Awad, 2017) studied the relative expression of mRNAs for some major inflammatory cytokines such as interleukin-6 (IL-6), tumor necrosis factor-alpha (TNF-alpha), and IFN-gamma (IFN-γ), as well as nuclear factor kappa-light-chain-enhancer of activated B cells p50 subunit (NF-κB p50), in broilers that received thyme-supplemented diets. The results revealed that dietary thyme supplementation resulted in reduced expression of pro-inflammatory cytokines. TNF-alpha expression was significantly inhibited in broilers that received thyme at doses of 5-8 g/kg. This indicates that thyme supplementation may exert an anti-inflammatory response in broilers. IL-6 expression was significantly reduced in broilers that received thyme at a dose of 8 g/kg. However, IL-6 expression was not significantly reduced in broilers that received thyme at doses of 2 and 5 g/kg. This may indicate that IL-6 is reduced in a dose-dependent manner. Moreover, the study indicated that thyme supplementation does not affect the expression of IFN-γ and NF-κB p50. This may indicate that thyme supplementation targets specific aspects of inflammation. Thyme supplementation also affects mucosal health in broilers. Hassan and Awad (2017)(Hassan & Awad, 2017) also studied the expression of mucin2 (MUC2), which is a major protein in the mucus layer in the intestines and is essential for maintaining the integrity of the mucus layer. The results revealed that MUC2 expression is significantly induced in broilers that received thyme at a dose of 8 g/kg. A similar but insignificant increase in MUC2 expression was also noted in broilers that received thyme at a dose of 2 g/kg. This may indicate that thyme supplementation may exert beneficial effects in maintaining the integrity of the mucus layer in the intestines. This may be beneficial for improved gut health in broilers. The modulation of inflammation and mucin2 may be important for understanding the beneficial role of thyme supplementation in improving gut health in broilers. Improved gut health may be essential for improved growth performance in broilers. Moreover, thyme extracts reduce the expression and activity of the COX-2 enzyme. This results in lower production of prostaglandins (specifically PG E2), which are responsible for pain, fever, and vasodilation in birds. Thyme also targets the LOX pathway, reducing the synthesis of leukotrienes. These molecules are potent inflammatory mediators that can cause airway constriction and attract immune cells (chemotaxis) that, if overactive, damage gut tissue(Zaazaa et al., 2022);(Basiouni et al., 2023). However, given the relatively small number of studies on gene expression in response to thyme supplementation, further research is needed to fully elucidate the broader molecular mechanisms at play. Studies exploring the effects of thyme on other key genes involved in immunity, metabolism, and digestion will provide a more comprehensive understanding of its potential benefits for poultry production.