Thyroid hormone rapidly induces hepatic LDL receptor mRNA levels in hypophysectomized rats

The effects of hypophysectomy and thyroid hormone treatment on the expression of the hepatic LDL receptor gene was studied in young male rats. Hypophysectomy lowered levels of LDL receptor mRNA and protein. It was found that increased levels of LDL receptor mRNA could be detected 30 min after giving hypophysectomized rats an intramuscular injection of 10 micrograms of L-triiodothyronine (T3) per 100 g of body weight. This dose of T3 also increased hepatic LDL receptor protein levels within 1 h. A T3 dose of only 0.25 microgram per 100 g increased LDL receptor mRNA levels more than threefold. The half-life of the hepatic LDL receptor mRNA was found to be about 30 min and was unaffected by T3. The data suggests that thyroid hormone acts physiologically to induce hepatic LDL receptor expression. This action may explain, in part, the hypocholesterolemic effect of the hormone.