Effects of Thyrotropin-Releasing Hormone and a Thyrotropin Releasing Hormone Analog on Growth and Selected Plasma Hormones in Lambs

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Abstract
An 8-wk growth trial was conducted to assess the effects of continuous infusion of thyrotropin-releasing hormone (TRH) and an active TRH analog 2 (the −1 *d−1 with 16 lambs/treatment. Both TRH and 2 decreased average daily gain (ADG; P<.Q1) and increased feed conversion (FC; P<.01) compared with saline infused controls. Average daily feed intake was not altered. Carcasses of lambs given TRH or 2 contained fewer kilograms of moisture (P<.05) and appeared to contain fewer kilograms of protein. Thyrotropin-releasing hormone and 2 increased thyroid gland weights (P<.05), but pituitary gland weights were not different. Plasma thyrotropin (TSH) concentrations were increased by both drugs compared with control lambs, peaking at 4 to 7 d after initiating infusion. However, by 14 d, TSH concentrations returned to control levels. Triiodothyronine (T3) and thyroxine (T4) were elevated by both drugs over the entire 8-wk trial, with peak levels reached at 10 d and maintained for the duration of the study. Both TRH and 2 increased prolactin over the entire period. Growth hormone levels were not altered by either drug. The effects of 2 infusion on growth trial performance, carcass composition and hormone profiles of growing lambs were very similar to TRH. The negative effects of TRH and 2 infusion on ADG, FC and carcass protein appear to be the result of elevated T3 and T4 levels. Copyright © 1983. American Society of Animal Science . Copyright 1983 by American Society of Animal Science

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