In newborn calves, injecting the short version of growth hormone‑releasing factor (GRF‑1‑29) makes the pituitary release more growth hormone (GH), but it doesn't change the downstream hormone IGF‑1 or blood sugar. A similar hormone, TRH, also boosts GH and other hormones, but the effect is weaker in slightly older calves.

Plasma somatotropin (GH) and somatomedin C (IGF1) concentrations were measured by radioimmunoassay in 3-dy old and 10-day old calves intravenously injected with growth hormone releasing factor (GRF) 1-44, GRF 1-29 or thyrotropin-releasing hormone (TRH). In 3 day old animals the increase in plasma GH concentration was GRF 1-44 dose-related (50, 100, 200 pmoles . kg(-1) of body wt). In four 10-day old calves injected with the lowest dose, the increase in plasma GH concentration was not different from that observed in four 3-day old animals treated with 200 pmoles . kg-1 of body weight. However, the response observed in four 3-day old calves injected with GRF 1-29 (50 pmoles . kg-1 of body wt) was not different from that observed following the same treatment in four 10-day old calves. In four 3-day old calves TRH (10 nmoles . kg-1 of body wt) induced a significant rise in plasma GH, prolactin (Prl), thyroxine (T4) and triiodothyronine (T3) concentrations. The same dose of TRH injected into four 10-day old calves elicited a similar rise in plasma T3 and T4 concentrations, but plasma GH and Prl increased less than in 3-day old animals. In three 3-day old or 10-day old calves born spontaneously before term (258-260 days of gestation), the increase in plasma Prl and GH concentrations following TRH was not different from that observed in mature calves of the same postnatal age. Neither GRF 1-44, GR 1-29 nor TRH elicited any significant change in plasma IGF1, insulin or glucose concentration in any group of calves.