Serum concentrations of thyroid hormones and activity of iodothyronine deiodinase in peripheral tissues of the house musk shrew, Suncus murinus.

Serum concentrations of thyroid hormones and the properties of iodothyronine deiodinase in tissues of the house musk shrew, Suncus murinus, were examined and compared with those of the rat. Serum concentrations of thyroxine (T4) and 3,3',5'-tri-iodothyronine (rT3) were higher, while the serum concentration of 3,5,3'-tri-iodothyronine (T3) was lower in the shrew than in the rat. Among liver, kidney, skeletal muscle, heart, brown adipose tissue (BAT), spleen, lung, testes and thymus homogenates of the shrew, T(4)5'-deiodinase (5'D) activity was highest in BAT, and rT3 5'D activity was highest in both the liver and BAT. Intermediate or low T3 5-deiodinase activity was noted in all tissues examined. Activity of 5'D for T4 and rT3 in liver and kidney was much lower, while that of BAT was much higher in the shrew than in the rat. Liver and kidney 5'D may be type-I and that of BAT may be type-II in the shrew, judging from its response to 6-n-propyl-2-thiouracil and iopanoic acid and substrate preference. Thus 5'D of the shrew was similar to that of the rat in type, but was different with respect to its activity in some peripheral tissues. This difference may have relevance to the low T3 state of the shrew.