The effect of CLA supplementation of feeds for sows in late pregnancy on thyroid hormone, total cholesterol and cholesterol fraction (LDL and HDL) concentrations in the serum of piglets was determined. The experiment was conducted on 8 lactating sows (Large White x Polish Landrace) divided into two groups. The sows were fed a complete granulated feed free access to water. From day 90 of pregnancy to delivery, the sows received a 4% addition of sunflower seed oil or CLA oil (EDENOR UKD 6010, Henkel). Samples of colostrum were taken 24 h after delivery. Milk samples were taken 3 h after the morning feeding on days 8 and 21 of lactation. On day 21, blood from the jugular vein was sampled from two piglets in each litter for determination of thyroid hormone, total cholesterol, HDL and LDL cholesterol. The addition of CLA to the rations of the experimental sows late in pregnancy increased the level of conjugated linoleic acid in colostrum in comparison with the control group (P<0.001). On days 8 and 21 of lactation the CLA content of milk was lower than in colostrum (P<0.001). Significantly higher levels of thyroxine and triiodothyronine (P<0.05), lower total cholesterol and HDL (P≤0.05) and LDL (P≤0.01) levels were found in the blood plasma of piglets in the experimental group in comparison with those in the control group. The obtained results show that conjugated linoleic acid contained in the colostrum of sows increases plasma thyroid hormone levels in the blood of piglets and lowers the levels of total cholesterol and cholesterol fractions.