Seasonal variations in thyroid hormone levels in free-living echidnas (Tachyglossus aculeatus)

Hulbert and Augee (1982) have suggested that the thyroid has little effect on energy metabolism in the echidna. In order to investigate whether thyroid status changes during hibernation, when metabolism drops dramatically, we measured levels of thyroid hormones in 31 free-living echidnas at various times during the year. Unlike eutherian hibernators, in which thyroid hormone levels may rise to seasonally high values in late hibernation, total and free T4 and T3 were all significantly depressed throughout hibernation. TT4 from nonhibernating echidnas was 11.8 ± 0.9 ng/ml (n = 23), confirming previously published values, but fell to half this level (5.9 ± 0.7 ng/ml, n = 8) during hibernation. By contrast to the low TT4 values, nonhibernating FT4, TT3, and FT3 values were similar to normal values for eutherian mammals. Differences in the seasonal pattern of variation in thyroid hormones between echidnas and hibernating eutherians may be due to differences in thyroid hormone transporting proteins.