Decreased pituitary growth hormone content in rats treated neonatally with high doses of L-thyroxine.

Growth hormone and thyrotrophic hormone content have been measured by specific radioimmunoassays in anterior pituitaries of 22 day old rats. These animals were injected with saline or very high doses of L-thyroxine during the neonatal period in order to induce the "neo-T4" syndrome. Growth of such animals is known to be affected. It was found that not only TSH but also GH content of the T4-treated animals was significantly lower than that of the saline-injected controls.