INVESTIGATION OF MECHANISMS MEDIATING THE INCREASE IN PLASMA-CONCENTRATIONS OF THYROID-HORMONES AFTER A MEAL IN YOUNG GROWING PIGS

These studies investigated a number of possible mechanisms which could mediate the increase in plasma concentrations of thyroid hormones after a meal in young growing pigs. It has been established that in animals fed one meal a day, an immediate rise in plasma 3,5,3'-tri-iodothyronine (T-3) and a slightly delayed increase in thyroxine (T-4) levels are followed by a more sustained peak in both hormones several hours later. The increase in thyroid hormones involves both total and free T-3 and T-4, and there is no change in plasma albumin, the high-capacity thyroid hormone-binding protein in the pig. It has also been shown that the immediate rise in plasma T-3 is not mediated either by an increase in plasma glucose concentration or by neural mechanisms associated with distension of the gastrointestinal tract. However, the finding that plasma T-3 increases rapidly after feeding in thyroidectomized animals maintained on a replacement dose of T-4 alone, indicates the source of T-3 to be non-thyroidal. It is concluded that the rise in plasma thyroid hormones after a meal depends on the energy content of the food but not directly on the circulating glucose levels. The immediate increases in plasma T-3 and T(4)are probably due largely to a redistribution of the hormonal pools, and peripheral 5'-monodeiodination of T-4 may also contribute significantly to the postprandial rise in T-3. The potential significance of these findings in relation to both the metabolic and growth-promoting effects of thyroid hormones is discussed.