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

Plasma concentrations of thyroxine (T4) and growth hormone (GH) were measured in the developing bandicoot from 21 through to 69 days post partum. Thyroxine concentrations increased from 7 ng mL(-1) at 21 days post partum to reach a maximum of around 40 ng mL(-1) approximately 40 days post partum. After this time, plasma T4 concentrations decreased until about 50 days post partum, when levels were not different from those in the adult (9.2 +/- 0.7 ng mL(-1), n = 10). In contrast, GH concentrations were greatest early in pouch life, with a maximum concentration of 88.2 ng mL(-1) at 24 days post partum, and thereafter declined to adult levels (4.9 +/- 0.9 ng mL(-1), n = 7) by about 60 days post partum. The temporal relationship between T4 and GH in the developing bandicoot is similar to that seen in developing eutherian mammals, but in the latter species, peak plasma T4 and the decline in GH occur before or soon after parturition, whereas in the bandicoot these events occur more than one month post partum. This comparison between eutherian mammals and a marsupial indicates that the timing of these endocrine changes correlates with key developmental or maturational changes rather than the time of parturition.