Plasma thyroxine levels of mammals: Desert and mountain

Plasma thyroxine levels were measured in twelve species of desert mammals ranging in altitude from the desert floor to montane forest regions. Rodent species from the low desert had lower circulating thyroxine levels than those species found at 1800 m elevation. Total thyroxine plasma levels were significantly related to metabolic rate and ecologic distribution. The low T 4 values of species restricted to the desert floor may play a role in adaptation to summer heat and to the scarcity of food and water.