Cold-induced changes in thyroid function in a poikilothermic mammal, the naked mole-rat

Cold acclimation induces very divergent responses in thyroid function in re ptiles and mammals reflective of their different thermoregulatory modes. Na ked mole-rats, unlike other small mammals, are unable to effectively employ endothermy and are operatively poikilotherms. We therefore investigated ch anges in their thyroid status with chronic cold exposure. Under simulated b urrow conditions, free thyroxine (T-4; 0.39 +/- 0.09 ng/dl) and thyroid sti mulating hormone (TSH; 1.12 +/- 0.56 mu IU/ml) levels fell within the repti lian range, one order of magnitude lower than mammalian levels. However, co ld induced typical mammalian responses: free T-4 levels (0.55 +/- 0.09 ng/d l) and thyroid follicular cell height were significantly greater. Although TSH levels (1.28 +/- 0.83 mu IU/ml) were not significantly elevated, thyrot rophs exhibited ultrastructural signs of increased secretory activity. Low thyroid hormone concentrations may contribute substantially to the unusual thermoregulatory mode exhibited by naked mole-rats.