Effects of thyroid hormone deiodination on regulation of thyroid axis in undernourished rats

The possible influence of hypothalamic and pituitary 5'-deiodinase II (5'-D-II) activity and 3,5,3'-triiodothyronine (T3) content on the modulation of thyroid-stimulating hormone (TSH) synthesis was studied. 1) Alterations in 5'-D-II activity and hypothalamic and pituitary T3 content produced by undernutrition were observed in fetal (21 days) and neonatal rats vs. controls. 2) After thyroidectomy, plasma TSH increased in both populations, undernourished and control, but pituitary TSH increased only in the former and not in the latter. The results obtained by giving small doses of thyroxine (T4; 0.5 micrograms/100 g body wt) to intact and thyroidectomized rats suggest a lower inhibitory effect by T4 on the pituitary in undernourished than in control rats. Although hypothalamic and pituitary 5'-D-II activity increased in both groups after thyroidectomy, the percentage increase was lower in undernourished vs. control rats, resulting in lower overall T3 content in tissues from undernourished animals. These studies on thyroid axis regulation show the in vivo regulation of TSH synthesis by hypothalamic and pituitary 5'-D-II activity and T3 content.