Dietary Silymarin (Silybum marianum) Supplementation Modulates the Adrenal, Thyroid, and Metabolic Hormone Axes in Growing Awassi Lambs: A 90-Day Feeding Trial

Abstract

Background. Intensively managed small ruminants in semi-arid environments are chronically exposed to oxidative and management-related stressors that activate the hypothalamic–pituitary–adrenal (HPA) axis and blunt anabolic hormone signalling. Silymarin, a flavonolignan complex extracted from Silybum marianum, combines antioxidant, anti-inflammatory, and hepatoprotective actions, yet its endocrine effects in lambs under field conditions remain poorly characterised. Objective. To evaluate the effect of two dietary levels of silymarin on serum cortisol, thyroid hormones (T3, T4, TSH), growth hormone (GH), and insulin in growing Awassi lambs over a 90-day period. Methods. Twelve male Awassi lambs (3–4 months; 26 ± 2 kg) were allocated to a completely randomised design with three treatments (n = 4): a basal diet without additive (T0), and the basal diet supplemented with 0.25 g (T1) or 0.75 g (T2) silymarin/head/day. Serum hormones were quantified by ELISA on days 45 and 90 and analysed by one-way ANOVA with Duncan’s multiple range test (P < 0.05). Results. No treatment effect was detected on day 45 except for insulin. By day 90, cortisol was significantly lower in both supplemented groups than in controls, with the largest reduction at 0.75 g/day. T3 and GH rose significantly in the 0.75 g group, whereas T4 and TSH were unaffected at either sampling point. Insulin increased in a dose-dependent manner and remained highest in the 0.75 g group at both time points. Conclusion. Silymarin at 0.75 g/head/day attenuates the adrenal stress response and enhances metabolic and anabolic hormone signalling in Awassi lambs while preserving thyrotropic feedback, supporting its use as a phytogenic feed additive and identifying an optimal dose window for confirmatory trials.