General and Comparative Endocrinology vol:77 issue:2 pages:212-220
The ontogeny of the effect of chicken growth hormone (c-GH) on the liver 5′ monodeiodination (5′-D) activity was studied in chickens starting from 14-day-old embryos until 5-week-old chickens. Ten micrograms of the purified hormone was ibjected intravenously and after 2 hr, blood and liver samples were taken. In all embryonic stages tested, c-GH stimulated the peripheral thyroxine (T4) to triiodothyronine (T3) conversion as demonstrated by the increased plasma T3 level and liver 5′-D activity while the plasma reverse T3 (rT3) level was decreased. In chicks 1 day after hatching, GH was still able to increase the plasma T3 level and the liver 5′-D activity. However, in 2-day-old chicks, this stimulation had completely disappeared, while the control value of the plasma T3/T4 ratio was considerably higher than the day before. The effect of c-GH injection gradually turned into a decrease of 5′-D activity and plasma T3 in 5- and 7-day-old chicks. Further on, no effect of c-GH could be found in 2- to 4-week-old growing chickens, but in 5-week-old GH-injected animals the plasma T4 and rT3 level decreased again, although no significant effect on T3 or liver 5′-D activity could be demonstrated.