Distribution of 125I-thyroxine in different organs and tissues of dietically obese rats.

The distribution of 125I-thyroxine (% dose/g tissue; tissue/plasma radioactivity ratio) was investigated in different tissues of 28-week-old obese Wistar rats. Obesity was induced by high-fat diet (HFD) and confirmed by carcass analysis; in heavy obese animals the relative and absolute fat content is increased twofold and threefold, respectively, compared to control rats fed on a low-fat diet (LFD). Heavy HFD rats exhibit diminished 125I-T4 distribution in the "slow pool" (fat tissue, muscle) and unchanged values in the "fast pool" (liver, kidney) in comparison with LFD rats with low body weight. The differences in distribution presented here are not caused by the diet per se, but they are the consequence of the obesity of the animal, because no differences in the 125I-T4 distribution were found in the 125I-T4 between HFD and LFD rats with relatively equal body weight and body composition. The reduced T4 distribution in the fat tissue of obese rats is discussed in connection with possibly decreased lipolysis in this tissue and possible causal participation in the beginning of obesity.