Both experimental hypo- and hyper-thyroidism exacerbate the adverse effects of chronic heat stress in broilers

1. The effects of hypo- and hyper-thyroidism in mitigating or exacerbating the negative changes of chronic heat stress (HS) in broilers were investigated. 2. Three-week-old broilers were distributed into six groups (n = 13 per group). Three groups were housed at ambient room temperature: control group (CN), propylthiouracil-treated group (AN) and thyroxine-treated group (TN). The other three groups were exposed to HS at 33 ± 1°C for 2 weeks: control heat stress (CH), propylthiouracil + heat stress (AH) and thyroxine + HS (TH). 3. Induced hypothyroidy significantly decreased cloacal temperature and body weight gain in the birds in both the normal and HS groups (AN, AH). Conversely, hyperthyroidy resulted in a significant elevation in cloacal temperature in the TN and TH groups and a significant decline in weight gain in the TH group. Hyperthyroidy exacerbated the HS-induced degenerative changes in jejunal mucosa and caused noticeable vascular changes. A significant increase in the expression levels of jejunal nutrient transporter genes was observed in the AH and TH groups. The hyperthyroidic state significantly upregulated the HSP70 expression level in the TH group and the reverse occurred with propylthiouracil (PTU) treatment in the AH group. 4. PTU supplementation to chicks reared under HS significantly decreased the triiodothyronine level, antibody (Ab) titre, and increased the heterophil-lymphocyte ratio. Furthermore, it induced higher hepatic glutathione peroxidase (GSH-Px) activity in the AN and AH groups and decreased the malondialdehyde content (MDA) in the AN group. Hyperthyroidy significantly increased triiodothyronine concentration, H/L ratio and decreased Hb concentration and Ab titres in the TH group. Additionally, this status increased the MDA content and decreased the GSH-Px activities. 5. In conclusion, manipulation of thyroid status is not a remedy to overcome the undesirable effects of HS in broilers.