Selenium is a key trace element in thyroid homeostasis, as it is a component of selenoproteins, including iodothyronine deiodinase, which convert thyroxine (T4) into the more biologically active triiodothyronine (T3). At the same time, selenium has a narrow “therapeutic window,” so it is important to find forms with higher bioavailability and lower toxicity. The aim of the study was to evaluate the effect of biogenic selenium nanoparticles (SeNPs), synthesized by “green” microbiological reduction of Na 2 SeO plantarum 3 with the probiotic culture Lactobacillus , on the concentration of free T3 and T4 and the activation coeffi cient (T3/T4×100) in the blood of Cobb-500 broiler chickens. The experiment used 200-day-old chickens divided into 5 groups (n=40): control; sodium selenite 0.30 mg Se/kg feed + probiotic; SeNPs 0.15, 0.30, and 0.45 mg Se/kg feed + probiotic. Blood samples were taken on days 21 and 42; free T3 and T4 were determined by ELISA (Granum test systems, Ukraine), and statistical signifi cance was assessed using Student's t-test (p<0.05). A dose-dependent increase in T3 and decrease in T4 was observed in all selenium-containing groups as early as day 21. On day 42, SeNPs at a dose of 0.45 mg/kg provided an activation coeffi cient of 16.40 compared to 5.98 in the control and 8.36 in the selenite group; at an equivalent dose of 0.30 mg/ kg, SeNPs increased T3 by approximately 40% and decreased T4 by 23% compared to selenite. The data obtained indicate an increase in T4→T3 conversion under the action of biogenic SeNPs and a cumulative eff ect between days 21 and 42. A dose of 0.30 mg Se/kg of feed is considered optimal for><0.05). A dose-dependent increase in T3 and decrease in T4 was observed in all selenium-containing groups as early as day 21. On day 42, SeNPs at a dose of 0.45 mg/kg provided an activation coeffi cient of 16.40 compared to 5.98 in the control and 8.36 in the selenite group; at an equivalent dose of 0.30 mg/ kg, SeNPs increased T3 by approximately 40% and decreased T4 by 23% compared to selenite. The data obtained indicate an increase in T4→T3 conversion under the action of biogenic SeNPs and a cumulative effect between days 21 and 42. A dose of 0.30 mg Se/kg of feed is considered optimal for practical use, providing a pronounced thyrotropic ef fect while minimizing the risk of excessive selenium accumulation; the “probiotic-nanoparticle” synergy is a promising platform for the creation of functional feed additives in poultry farming. The T3/T4 ratio can serve as a biomarker of the eff ectiveness of selenium supplementation in broilers and for further optimization of dosage.

Keywords: selenium nanoparticles, green synthesis, probiotics, thyroid hormones, broiler chickens, triiodothyronine, thyroxine, Lactobacillus plan tarum , activation coefficient, deiodinases.