Concentrations of thyroid hormones and iodothyronine binding proteins in serum of the koala (Phascolarctos cinereus)

Objective To examine circulating total and free thyroid hormone (T₃ and T₄) concentrations, determine serum iodothyronine binding characteristics and estimate thyroid stimulating hormone (TSH) activity in sera of coastal and inland koalas.
Design A prospective study.
Procedure Koala serum T₃ and T₄ were measured by radioimmunoassay. T₄ binding parameters were determined by radioligand binding and electrophoresis. Koala TSH values were determined by bioassay.
Results Mean total T₄ concentrations were 3.2 ± 2.1 nM although values were significantly higher in inland-dwelling females in comparison to coastal-dwelling males. Free T₄ was 3.3 ± 2.1 pM. Total and free T₃ were 0.4 ± 0.2 nM and 1.4 ± 0.9 pM respectively, although these values were at the lower end of the assay detection limit and should be viewed with reservation. Electrophoresis of ¹²⁵I]-T₄-labelled serum revealed only two proteins of electrophoretic mobility similar to human transthyretin (TTR) and albumin. Scatchard analysis of T₄ binding to serum gave a curvilinear plot, which could be resolved into two binding sites with affinities identical to that of TTR and albumin but both of low concentration. The bioactivity of the TSH present in the sera was measured using a cell line (JP09) transfected with the human TSH receptor. The mean level of stimulation found in the sera corresponded to a bovine TSH activity of less than 10 mU/L.
Conclusion These results suggest that the serum concentrations of free and total thyroid hormones in koalas are low compared to other marsupials and very low compared to eutherian mammals. The mechanism of maintenance of euthyroidism in this species remains to be determined.