Thyrotropin-Releasing Hormone Stimulates Body Weight Gain and Increases Thyroid Hormones and Growth Hormone in Plasma of Cockerels

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Abstract
The effect of TRH on growth in 4 wk old cockerels was examined in 2 separate experiments. Daily injection of TRH via the brachial vein stimulated growth in 4 wk old cockerels over 17 days of treatment in the 1st experiment and over 25 days in the 2nd. In the 1st experiment, TRH at 1.0 .mu.g and 10.0 .mu.g/bird caused significant (P < 0.05) increases of 12.0% and 12.4%, respectively, in growth rate, whereas in the 2nd experiment, only the 1.0 .mu.g/bird level of TRH caused an increase (P < 0.05). In each experiment, the increase in body weight gain was not TRH dose dependent, and neither feed consumption nor feed efficiency was affected. Possible involvement of pituitary hormones in TRH-stimulated growth in cockerels was studied in a separate experiment, and the effects of TRH on plasma triiodothyronine [T3], thyroxine [T4] and growth hormone [GH] were examined. TRH was given i.v. at 0.1, 1.0, and 10.0 .mu.g/bird daily for 5 days, and plasma T3, T4 and GH concentrations were measured 15, 60, and 180 min postinjection by radioimmunoassay on days 1, 3 and 5. The responses of T3 and T4 to TRH were greatest on day 1, were diminished by days 3 and 5, and were not dose-related. Significant, not dose-related elevations of plasma GH concentrations were obtained at all doses of TRH. TRH may have the ability to promote a significant increase in body weight gain in 4 wk old cockerels, and the stimulatory effects may be mediated through GH and/or thyroid hormones.