Thyroid function in newborn lambs: influence of prolactin and growth hormone

Lambs originating from Suffolk, Milksheep and Texel crossbreeds were injected with saline, 500 micrograms ovine prolactin or 500 micrograms 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 micrograms 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 less than 0.01) and to 6.51 +/- 0.42 nmol/l after 2 h (P less than 0.05). At both times these values were significantly (P less than 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 less than 0.05) and to 5.10 +/- 0.45 nmol/l after 2 h (P less than 0.01; values were also significantly different from saline controls at P less than 0.05 and less than 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.(ABSTRACT TRUNCATED AT 250 WORDS)