Lambs originating from Suffolk, Milksheep and Texel crossbreeds were injected with saline, 500 μg ovine prolactin or 500 μg ovine GH within 30 min of parturition (n = 10). Birth weight was negatively correlated with plasma concentrations of reverse tri-iodothyronine (rT3) but not with thyroxine (T4), free T4 (expressed as the free thyroxine index) or cortisol. At birth, T3 and T4 plasma concentrations were high and remained high during the 3-h observation period. After 3 h a significantly lower rT3 concentration was found. The cortisol concentration at birth was also high (100–400 nmol/l) but decreased rapidly to basal values after 1 h. An injection of 500 μg prolactin after parturition did not influence the hormonal parameters studied except for rT3 where, after 2 h, lower plasma concentrations were found compared with controls.
Growth hormone raised T3 levels from 4·80 ± 0·44 (s.e.m.) nmol/l at birth to 6·74 ± 0·42 nmol/l at 1 h after birth (P < 0·01) and to 6·51 ± 0·42 nmol/l after 2 h (P < 0·05). At both times these values were significantly (P < 0·001) different from saline-injected controls. GH decreased rT3 from 6·77 ± 0·71 nmol/l at birth to 5·42 ± 0·54 nmol/l after 1 h (P < 0·05) and to 5·10 ± 0·45 nmol/l after 2 h (P < 0·01; values were also significantly different from saline controls at P < 0·05 and < 0·005 respectively). At the same time total and free T4 concentrations were increased. No influence of prolactin or GH injection on plasma cortisol concentrations was seen. It is concluded that GH may play an important role in the maturation of thyroid functions during the perinatal period of lambs by increasing T4 secretion and by increasing the conversion of T4 to T3 and decreasing the T4 to rT3 conversion rates.