Plasma concentrations of thyroxine and growth hormone in the developing marsupial bandicoot, Isoodon macrourus

Abstract

Plasma concentrations of thyroxine (T ₄ ) and growthhormone (GH) were measured in the developing bandicoot from 21 through to 69days post partum. Thyroxine concentrations increased from 7 ng mL^–1 at 21 days post partum to reach a maximum ofaround 40 ng mL ^–1 approximately 40 days postpartum. After this time, plasma T ₄concentrations decreased until about 50 days post partum, when levels were not different fromthose in the adult (9.2 ± 0.7 ng mL ^–1 ,n=10). In contrast, GH concentrations weregreatest early in pouch life, with a maximum concentration of 88.2 ng mL^–1 at 24 days post partum, and thereafter declinedto adult levels (4.9 ± 0.9 ng mL ^–1 ,n= 7) by about 60 days post partum. Thetemporal relationship between T₄ and GH in thedeveloping bandicoot is similar to that seen in developing eutherian mammals,but in the latter species, peak plasma T₄and the declinein GH occur before or soon after parturition, whereas in the bandicoot theseevents occur more than one month post partum. This comparison betweeneutherian mammals and a marsupial indicates that the timing of these endocrinechanges correlates with key developmental or maturational changes rather thanthe time of parturition.