Relationships between plasma thyroxine concentrations and the responses of newborn lambs to hypothermia.

Abstract

Animal Production in Australia RELATIONSHIPS BETWEEN PLASMA THYROXINE CONCENTRATIONS AND THE RESPONSES OF NEWBORN LAMBS TO HYPOTHERMIA I.W.CAPLE* and G.F.NUGENT* Newborn lambs have higher plasma thyroxine (T4) concentrations than their mothers if the ewes have adequate iodine nutrition. Lambs born with goitre due to iodine deficiency may have plasma T4 less than their mothers (Andrewartha et al (1980), and are particularly susceptible to cold stress. Little attention has been given to the role of thyroid hormones in the response of newborn lambs to cold (Alexander 1970), and experiments were conducted to determine if any relationships existed between this response and plasma T4. In 16 pregnant crossbred ewes the foetal lambs were operated on between 90 and 125 days of gestation. A catheter was placed in the foetal carotid artery to enable blood samples to be collected. In 13 lambs the thyroid glands were removed and these thyroidectomised lambs were supplemented with either 0, 2.5, 5, 8 or 10 1-19 T4/day to provide a group of lambs with a range of plasma T4 at birth. Daily injections of between 8 and 10 pg T4/day maintained plasma T4 of the thyroidectomised lambs at similar levels to those of 3 control lambs. All 0 to 61 suck the born in in ewes 13 thyroidectomised lambs were born alive us/l. Six of the lambs died shortly after ewe. The catheterized control lambs, 10 a shed had plasma T4 ranging from 36 to 98 ranged from 30 to 43 pg/l. and had plasma T4 ranging from birth, and 7 could stand and lambs born in a paddock, and 10 pg/l at birth. The plasma T4 The resistance of the 30 suckled in a progressively cooled water bath to 24Oc in 30 minutes and then to 20�C ~arnbs were removed from the bath when rewarmed in air. newborn lambs to hypothermia was determined where the temperature was reduced from 38Oc over the next 30 minutes (Slee et al 1980). rectal temperature decreased to 35oC, and There was a significant positive relationship between plasma T4 concentration and the time taken for the rectal temperature of lambs to decrease to 350C (r=0.73 p<O.OOl). The maximum rate of decrease in rectal temperature in the bath (summit metabolism) was not significantly related to plasma T4 concentration. The rate at which rectal temperature increased in air was positively relased to plasma T4 (p<O.OOl) and the times taken for rewarming were prolonged if plasma T4 was less than 50 pg/l. Plasma glycerol concentrations increased during hypothermia and were taken to indicate catabolism of brown fat: the increases were positively related to plasma T4 (p<O.OOl). It would appear that newborn lambs with plasma T4 less than 50 vg/l may have functional hypothyroidism with increased susceptibility to hypothermia. A diagnosis of hypothyroidism in newborn lambs can be made if their plasma T4 is less than that of their mothers or less than 50 us/l. ALEXANDER, G. (1970). Biol. Neonate == : 37. 15 ANDREWARTHA, K.A., CAPLE, I.W., DAVIES, W.D. and MCDONALD, J.W. (1980). Aust. Vet. J. 25 : 18. SLEE, J., GRIFFITHS, G. and SAMSON, D.E. (1980). Res. Vet. Sci. zg : 275. * Department of Agriculture, Attwood Veterinary Research Laboratory, Mickleham Road, Westmeadows, Vic- .3047- 657