A Comparative Study of Thyroid Function in Monotreme, Marsupial, and Eutherian Mammals

This study reports information on several aspects of thyroid function in a monotreme (Tachyglossus aculeatus), some marsupial (Isoodon macrourus, Perameles nasuta, and Macrotis lagotis), and eutherian mammals (Oryctolagus cuniculus and Rattus norvegicus). Thyroidal uptake of I125 was greatest in the eutherians and lowest in the echidna; however, the release rate of I125 from the thyroid (% per day) was greatest in the marsupials. Serum concentrations of total thyroxine were 15.7, 22.0, 57.9, and 64.1 nmol · l⁻ⁱ in the echidna, the bandicoot, the rabbit, and the rat, respectively. An in vitro assessment of "free" thyroxine suggested that it was a relatively greater proportion of the total hormone concentration in both the echidna and the bandicoot than in the rabbit. Serum levels of total triiodothyronine were 0.7, 1.5, 6.9, and 3.5 nmol · l⁻ⁱ, respectively, for the same four species. In both the rabbit and the bandicoot, thyroidectomy resulted in a significant reduction in metabolic rate; however, in the echidna, there was no significant reduction in metabolism following thryroidectomy. The role of thyroid in controlling the metabolism of these different mammalian groups is discussed.