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The Effect of Environmental Temperature on Concentrations of Thyroxine and Triiodothyronine After Thyrotropin Releasing Hormone in Steers¹

Oklahoma Agricultural Experiment Station, Stillwater 74078

Abstract

Twenty-four Angus x Hereford steers (155 ± 4 kg) were used to examine thyroid function during exposure to ambient temperatures of 4, 18 and 32 C. Jugular cannulae were inserted after steers were acclimated to individual stalls in environmentally controlled chambers at 18 C for 3 d. The day following cannulation, ambient temperatures were changed 2 C/h for 7 h and serum samples were collected hourly. After steers were exposed to either 4, 18 or 32 C for 1 and 72 h, thyrotropin releasing hormone (TRH; 50 µg, iv) was rapidly infused. Serum samples were collected hourly for 8 h after each treatment with TRH and every 8 h for 3 d between treatments. Rectal temperatures and respiratory were greater (P<.05) in steers exposed to 32 C compared with steers at 4 C. During the change in environmental temperature, the concentrations of thyroxine (T₄) and triiodothyronine (T₃) over time tended (P<.10) to decrease in steers exposed to 32 C compared with those at 4 C. Concentrations of T₄ and T₃ after the second treatment with TRH were significantly less in steers exposed to 32 C compared with those at 4 C. The response of T₄ to TRH was reduced (P<.01) after the second treatment with TRH compared with the first for steers exposed to all three temperatures, whereas, the response of T₃ was reduced (P<.05) after the second treatment with TRH only in steers exposed to 32 C. These data suggest that exposure of steers to elevated temperature for 3 d reduces the response of T₃ and T₄ to treatment with TRH and that the response of T₄ is reduced after a second injection of TRH 3 d after the first treatment.

¹ Journal article 4255 of the Agr. Exp. Sta., Oklahoma State Univ., Stillwater. Supported by ARS Cooperative Agreement #58-7B30-8-45.

² Present address: Dept. of Anim. and Vet. Sci., Univ. of Maine, Orono 04469.

³ Anim. Sci. Dept., Oklahoma State Univ., Stillwater 74078.