Environmental heat stress modulates thyroid status and its response to repeated endotoxin challenge in steers

The objective of this study was to evaluate in cattle, the effects of acute exposure to a heat stress (HS) environment on the status of the pituitary (thyrotropin, TSH)-thyroid (thyroxine, T₄)-peripheral tissue T₄ deiodination (type 1 5'-deiodinase [D1]; triiodothyronine [T₃]; reverse-triiodothyronine [rT₃]) axis, and the further response of this pituitary-thyroid-peripheral tissue axis (PTTA) to perturbation caused by the induction of the proinflammatory innate immune state provoked by the administration of gram-negative bacteria endotoxin (lipopolysaccharide [LPS]). Ten steers (318 ± 49kg body weight) housed in controlled environment chambers were subjected to either a thermoneutral (TN: constant 19°C) or HS temperature conditions (cyclical daily temperatures: 32.2°C-40.0°C) for a total period of 9d. To minimize the effects of altered plane of nutrition due to HS, steers in TN were pair-fed to animals in HS conditions. Steers received 2 LPS challenges 3d apart (LPS1 and LPS2; 0.2μg/kg body weight, intravenously, Escherichia coli 055:B5) with the first challenge administered on day 4 relative to the start of the environmental conditioning. Jugular blood samples were collected at 0, 1, 2, 4, 7, and 24h relative to the start of each LPS challenge. Plasma TSH, T₄, T₃, and rT₃ were measured by radioimmunoassay. Liver D1 activity was measured in biopsy samples collected before the LPS1 (0h) and 24h after LPS2. Before the start of LPS1, HS decreased (P < 0.01 vs TN) plasma TSH (40%), T₄ (45.4%), and T₃ (25.9%), but did not affect rT₃ concentrations. In TN steers, the LPS1 challenge decreased (P < 0.01 vs 0h) plasma concentrations of TSH between 1 and 7h and T₄ and T₃ at 7 and 24h. In HS steers, plasma TSH concentrations were decreased at 2h only (P < 0.05), whereas plasma T₃ was decreased at 7 and 24h (P < 0.01). Whereas plasma T₄ concentrations were already depressed in HS steers at 0h, LPS1 did not further affect the levels. Plasma rT₃ concentrations were increased in all steers at 4, 7, and 24h after LPS1 (P < 0.01). The patterns of concentration change of T₄, T₃, and rT₃ during LPS2 mirrored those observed in LPS1; the responses in plasma TSH were of smaller magnitude than those incurred after LPS1. The LPS challenges reduced (P < 0.01) hepatic activity of D1 in all animals but no differences were observed between steers subjected to TN or HS environment. The data are consistent with the concept that acute exposure of cattle to a HS environment results in the depression of the pituitary and thyroid components of the PTTA, whereas a normal capacity to generate T₃ from T₄ in the liver is preserved. The data also suggest that LPS challenge further suppresses all components of the PTTA including liver T₃ generation, and these PTTA perturbations are more pronounced in steers that encounter a HS exposure.