Changes in parameters of growth hormone and thyrotrophic hormone, and of thyroid function, during the early postnatal period in the rat

Abstract

The body length, and the weight of the body, liver, kidney and brain have been measured daily in rats during the first 10 days after birth. The plasma and pituitary growth hormone and thyrotrophic hormone levels were also determined, as well as the thyroidal 127I content and the plasma PBI. This observation period comprises a critical stage during which administration of large doses of thyroid hormones result in a permanent derangement of the thyroid-pituitary interrelations, and impairment of body growth and pituitary GH economy. The rate of growth of body, liver and kidney has been found to decrease significantly from day 7th to 9th of post-natal age, later to increase again. The pattern of the changes observed in the plasma and pituitary GH levels during the same period might well account for the alterations in growth patterns. The rate of growth of the brain, however, is not decreased during this stage, and appears to be independent of the changes in GH economy. No clearcut pattern of changes was observed in plasma TSH level; the pituitary TSH and thyroidal 125I contents increased progressively during the entire observation period. Plasma from suckling rats often contained high concentrations of non identified iodinated compounds, which were not thyroid hormones. Results are discussed in terms of the possible relationships between thyroid hormone and GH economy during a critical developmental period.