Mitochondrial binding of thyroxine as a function of thyroxine concentration (1.0×10−5M to 6.0×10−5M) was studied in the livers of normal control, hyperthyroid and hypothyroid rats with and without cold exposure at 2–4° C. Binding studies were carried out using non-labeled L-thyroxine and measuring the ultra-violet absorbancy of thyroxine left after the separation of mitochondria from the incubating medium. In mitochondria from the normal rat, a linear relationship between the amount of thyroxine bound and the concentration in the medium was observed. In mitochondria of rats cold-exposed for 5 hours, there appeared to be a smaller rate of increase in thyroxine binding at the higher concentrations. In the case of rats refrigerated for 8 hours, the uptake of thyroxine was maximal at 3×10−5M. The same linear relationship as for normal controls was found with mitochondria from hyperthyroid rats and 5 hours of cold-exposure did not exert any influence on this relationship. In mitochondria from hypothyroid rats, the relationship between thyroxine concentration and mitochondrial binding was similar to that for normal, coldexposed animals. The reduced rate of increase of bound thyroxine with increasing thyroxine concentration in mitochondria of both refrigerated and hypothyroid rats is interpreted as indicating a decrease in the number of available binding sites through their partial pre-saturation with thyroid hormone in vivo.