Carnitine metabolism in thyroid hormone treated rats and mice

This study was undertaken to investigate the changes in carnitine metabolism in rats and mice injected with T4 for 3 days and 10 days, respectively, and in rats fed a T3 and T4-supplemented diet for 6 weeks. Thyroid hormone administration brought about a significant increase in urinary excretion of total carnitine. In T3 + T4-treated rats urinary esterified-carnitine to free-carnitine ratio increased significantly in the later phase of administration. Carnitine pool size in the body was significantly decreased in both T4-injected mice and T3 + T4-fed rats. In the latter animals, this decrease was due to the reduced carnitine contents in organs other than the liver, especially in skeletal muscle. The amount of carnitine synthesized by control and T3 + T4-treated rats was calculated from the data on carnitine intake, urinary carnitine excretion and carnitine pool size in the body over the 6-week period. Values obtained were 66.2 +/- 3.2 (mean +/- SEM) mumol/rat and 28.5 +/- 4.9 mumol/rat, respectively, and the difference was significant (p less than 0.05). These results indicate that carnitine synthesis is depressed by thyroid hormone, however, some possibilities that thyroid hormone may increase carnitine synthesis were also discussed.