TABLE 4.

Overview of heat stress effects on chicken hormonal levels.

Class	Heat stress effect	Pros	Cons	References a
Hormonal levels	Hypothalamic-pituitary-adrenal axis activation → ↑ circulating glucocorticoids (e.g., corticosterone)	—	↓ growth potential	Decuypere and Buyse (1988) , Geraert et al. (1996b) , Yunianto et al. (1997) , Dong et al. (2007) , Yuan et al. (2008) , Quinteiro-Filho et al. (2010) , Quinteiro-Filho et al. (2012) , Renaudeau et al. (2012), Rhoads et al. (2013), Rajaei-Sharifabadi et al. (2017) , Lu et al. (2019) , Beckford et al. (2020) , Gonzalez-Rivas et al. (2020), Hirakawa et al. (2020) , Wasti et al. (2020) , Chauhan et al. (2021), Ma et al. (2021)
			↓ protein synthesis and ↑ protein breakdown in skeletal muscles → ↓ lean tissue yield
			↑ fat deposition
			↓ immunocompetence → ↑ infectious susceptibility and health care costs
			↓ GI barrier
	Hypothyroid-like state	↓ basal metabolism and thermogenesis → ↓ metabolic heat generation	↓ growth potential	Huston and Carmon (1962) , de Andrade et al. (1977) , Dale and Fuller (1980) , Decuypere and Buyse (1988) , Geraert et al. (1996b) , Yunianto et al. (1997) , Sohail et al. (2010) , Renaudeau et al. (2012), McNabb and Darras (2015) ; Rajaei-Sharifabadi et al. (2017) , Chen et al. (2018), Beckford et al. (2020) , Gonzalez-Rivas et al. (2020)
			↑ carcass adiposity
					↓ egg production and eggshell quality

Upward arrow (↑), increase; downward arrow (↓), decrease; rightward arrow (→), consequence/degeneration.

^aInclude studies on non-avian species that have exhibited comparable heat stress effects and responses to those observed in chickens. Studies with the focus on chickens or poultry are highlighted in bold.