Effect of feeding level and thyroxine on adipose tissue development and growth in postnatal lambs

We investigated the influence of exogenous thyroxine (T-4) administration in conjunction with level of feeding on adipose tissue and liver growth in postnatal lambs. Pairs of lambs were fed either 100 g (i.e. low fed) or 200 g (i.e. high fed) of milk powder per litre of reconstituted milk replacer over the first month of life. Half of the pairs of lambs were fed a bolus dose of T-4 (15 mg (kg body weight)(-1)) daily until 8 days of age. Perirenal adipose tissue and hepatic tissue were sampled at either 8 or 35 days of age. High fed lambs grew faster, possessed more adipose tissue and had larger livers than low fed lambs at 8 and 35 days of age. T-4 administration resulted in a lower thermogenic activity (i.e. GDP binding) in adipose tissue at 8 days of age in low, but not high fed lambs. There was no difference between groups in colonic temperature or oxygen consumption. Between 17 and 35 days of age high fed lambs previously treated with T-4 had lower daily milk consumption than untreated siblings, but still attained the same growth rate. Plasma insulin-like growth factor-I concentrations were greater in high than low Fed lambs, a relationship that was not influenced by T-4 treatment. In adipose tissue, iodothyronine 5' deiodinase activity was not influenced by T-4 administration and was greater in high than low fed lambs. Hepatic iodothyronine 5' deiodinase activity was not influenced by T, administration in low fed lambs, but was reduced by T-4 in high fed lambs. In conclusion, T-4 administration over the first 8 days of life can accelerate the rate of decline in thermogenic activity of uncoupling protein-1. This effect is not observed when the level of feeding is increased. Following withdrawal of T-4 treatment, high fed T-4-treated lambs were able to maintain the same growth rate as untreated lambs despite having a lower food intake.