Thyroid parameters and metabolic adaptation to cold in rats fed a low-bulk thyroxine-free diet

Abstract

Rats fed a purified low-bulk thyroxine-free diet (T₄-F) showed an increased sensitivity to noradrenaline similar to that in rats fed Master laboratory cubes. They also showed hypertrophy of the heart, liver, kidney, and adrenals, and reduced growth of the soleus and gastrocnemius muscles. However, thyroid weight and resting metabolic rate remained normal in cold-acclimated T₄F-fed rats whereas both parameters increased in cold-acclimated rats fed Master laboratory cubes. These observations are taken as evidence that thyroid hypertrophy and increased resting metabolic rate are diet-dependent and unnecessary for metabolic adaptation to cold. The responsible factor or factors in the diet remain to be determined.