The quantitative distribution of I-containing compounds has been studied in the thyroid and different tissues of intact rats, equilibrated isotopically with I 125 and exposed to 4-10°C for two weeks. It was found that the T 3 /T 4 ratio was increased in extrathyroidal samples. The T 3 /T 4 ratio in the intrathyroidal pool of "free" iodothyronines also tended to increase. In the liver, but not in the kidney, of cold-exposed rats, the concentrations of T 3 was higher, and the concentration of T 4 the same, or lower. The α-GPD activity increased in the liver, but not in the kidney of similarly treated animals. A positive correlation was found between α-GPD activity and T 3 concentration, but not between α-GPD and the concentration of T 4 . These and previous findings support the idea that conversion of T 4 to T 3 plays an important role in the expression of a biological effect of T 4 .
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Bernal et al. (1975) studied this question.