Differential effects of experimental hyperthyroidism on declined immunity of broiler chicken

Thyroid hormones (THs) are involved in the development of lymphoid organs and regulation of immune function in birds. However, their role as an immune‐modulator in the hyperthyroid state is still debatable. To explore the interrelationship of thyroxine (T₄) and the immune system, chicks were divided into three groups. Group I was comprised of control birds, who received the basal diet while group II and III were given diets supplemented with 5 μg and 10 μg thyroxine/kg feed, respectively, from 15 to 28 days of age. Cell‐mediated immune response was evaluated through in vitro abdominal macrophage phagocytosis assay, macrophage nitric oxide (NO) production, heterophil‐to‐lymphocyte (H:L) ratio and delayed‐type hypersensitivity response against phytohemagglutinin (PHA). Humoural immune response was assessed through serum IgG and IgM antibody production against sheep red blood cells (SRBCs) and antibody production against infectious bronchitis virus (IBV). Sampling was carried out at 7, 14 and 21 days of treatment. Results have shown higher levels (p < .001) of circulating T₄ in both treatment groups compared to the control group. There was a lower (p < .05) macrophage engulfment percentage, an increase in H:L ratio (p < .001) in treated birds, while their NO production remained higher (p < .05) in thyroxine supplemented groups after bacterial lipopolysaccharide stimulation. The humoural immune response revealed a significant decline (p < .001) in IgG, IgM antibody production against SRBCs but IBV circulating antibodies increased with age. In conclusion, hyperthyroidism has a strong co‐relation with decreased immune performance of birds.