Thyroidectomized rats have been injected daily with 125I labelled L-thyroxine (T4*) and, once isotopic equilibrium was attained, divided into cold-exposed (4–10°C) and control (21–24°C) groups, the daily T4* administration being continued till the end of the experiment. Fourteen days after onset of cold exposure, the total I* of different organs and of the carcass was determined and the tissues submitted to extraction and paper chromatography for the separation of T4 and T4-derived I-containing compounds. The activity of intramitochondrial α-glycerophosphate dehydrogenase (α-GPD) was measured in kidneys and liver. It was found that the total amount of I* was intensely decreased in all samples from cold-exposed animals. The proportion of this I* which was non-extractable was the same, in all tissues, for cold-exposed and control rats. The % of extractable tissue radioactivity in the form of T4 was decreased, and that found as T4-derived T3 was increased, in all samples from cold-exposed animals.
The T3/T4 ratio was increased more than two-fold in all tissues studied. The concentration of T4 decreased significantly in all tissues, whereas the concentration of T3 in tissues of cold-exposed rats did not decrease. It actually increased in kidneys and lungs, and remained the same in liver and carcass. Despite the decrease in the concentration of T4 in the kidneys, α-GPD activity was increased in this tissue, where the concentration of T3 was increased. No change in the α-GPD activity was found for the liver, where the concentration of T3 was the same for cold-exposed and control rats.
Thus, it appears likely that the conversion of T4 to T3 is increased by the exposure to cold of thyroidectomized rats on a constant dose of T4. α-GPD activity in a given tissue appears to be more closely related to the concentration of T3, than to that of T4.