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

Plasma concentrations of thyroxine (T-4) and growth hormone (GH) were measu red in the developing bandicoot from 21 through to 69 days post partum. Thy roxine 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. A fter this time, plasma T-4 concentrations decreased until about 50 days pos t 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 i n pouch life, with a maximum concentration of 88.2 ng mL(-1) at 24 days pos t 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 T-4 an d GH in the developing bandicoot is similar to that seen in developing euth erian mammals, but in the latter species, peak plasma T-4 and the decline i n GH occur before or soon after parturition, whereas in the bandicoot these events occur more than one month post partum. This comparison between euth erian mammals and a marsupial indicates that the timing of these endocrine changes correlates with key developmental or maturational changes rather th an the time of parturition.