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Untreated maternal hypothyroidism (hypoT) has serious consequences in offspring development that may result from the effect on lactation of maternal metabolism dysfunction. We studied the effects of prolonged propylthiouracyl (PTU)-induced hypoT (0.1% PTU in drinking water starting 8 days before mating until day 21 of pregnancy or for 30 days in virgin rats) on liver and mammary lipid metabolism and serum lipid concentrations. In virgins, hypoT reduced hepatic mRNAs associated with triglyceride (TG) and cholesterol synthesis (including fatty acid synthase and 3-hydroxy-3-methylglutaryl coenzyme A reductase), and induced lobuloalveolar mammary development. Thyroid hormones influence all major metabolic pathways. Their most obvious and well-known action is an increase in basal energy expenditure through actions on protein, carbohydrate, and lipid metabolism. With specific regard to liver lipid metabolism, thyroid hormones stimulate fatty acid and cholesterol synthesis (1, 2), increase mobilization of plasma cholesterol and triglycerides (TGs) (3, 4), and stimulate fatty acid and cholesterol degradation (5, 6). Disturbances in thyroid function are commonly associated with alterations in plasma lipid levels. Experimental hypothyroidism (hypoT) induced by propylthiouracyl (PTU) treatment is characterized by the accumulation of plasma LDL cholesterol, and decreased VLDL and plasma TGs (7), generally reflecting reduced binding activity of the hepatic LDL receptor (LDLR), which can be normalized after substitution therapy with thyroid hormone (3,8).Pregnancy is a state of dynamic changes in metabolism and nutrient utilization. The insulin-resistant condition and the increase in plasma estrogen levels occurring during late pregnancy are the main factors responsible for the development of a state of maternal hypertriglyceridemia that has been extensively studied in humans and rats (9)(10)(11). This condition benefits the progeny in two ways. First, it supplies essential fatty acids that are critical to normal fetal development and that circulate primarily esterified and associated with lipoproteins. A linear correlation between maternal and fetal plasma TGs has been described that has an important implication in newborn weight (12, 13). Second, it contributes to milk synthesis in preparation for lactation, providing circulating TG in the form of lipoprotein to the mammary gland (MG) for milk lipid synthesis (9).It has been demonstrated that the induction of hypothyroidism in dairy cows suppresses milk production during the treatment period (14). On the other hand, ad-

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