Chlortetracycline affects plasma growth hormone and thyroid hormones in beef steers after challenge with thyrotropin releasing hormone plus growth hormone releasing hormone

This study determined the effects of a sub-therapeutic level of chlortetracycline (CTC) in growing beef steers fed limited or adequate dietary protein on plasma concentrations of growth hormone (GH) and thyroid hormones before and after an injection of thyrotropin releasing hormone (TRH) + GH releasing hormone (GHRH). Thirty-two steers (Avg BW = 285±3 kg) were assigned to a 2 x 2 factorial arrangement of either 10% or 13% crude protein diets (70% concentrate, ad libitum intake) and either carrier or carrier + 350 mg CTC daily diet top dress. At 56 d, steers were injected via a jugular cannula (1.0 ug/kg BW TRH + 0.1 ug/kg BW GHRH in 10 mL saline) at 0800. Blood samples were collected from -30 to 360 min after injection. At 84 d, the steers were killed and the pituitary and samples of liver were collected and analyzed for 5′-deiodinase activity. CTC attenuated the GH response to injection by 26% for both area under the response curve (P < 0.03) and peak response (P < 0.10). CTC attenuated the thyroxine response for area under the curve by 12% (P < 0.08) and peak response by 14% (P < 0.04). Type II deiodinase activity in the pituitary was 36% less (P < 0.02) in CTC-fed steers compared to steers not fed CTC. Results suggest that feeding sub-therapeutic levels of CTC to growing beef cattle may influence tissue growth regulation via the pituitary.