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 (T3) and a slightly delayed increase in thyroxine (T4) levels are followed by a more sustained peak in both hormones several hours later. The increase in thyroid hormones involves both total and free T3 and T4, 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 T3 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 T3 increases rapidly after feeding in thyroidectomized animals maintained on a replacement dose of T4 alone, indicates the source of T3 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 T3 and T4 are probably due largely to a redistribution of the hormonal pools, and peripheral 5'-monodeiodination of T4 may also contribute significantly to the post-prandial rise in T3. The potential significance of these findings in relation to both the metabolic and growth-promoting effects of thyroid hormones is discussed.