The aim of the investigation was to study the effect of dietary conjugated linoleic acid (CLA) on the concentrations of lipids, fatty acids and iodothyronines: thyroxine (T₄), triiodothyronine (T₃), reverse-triiodothyronine (rT₃), free T₄ (FT₄) and free T₃ (FT₃) in blood plasma of pigs. The experiment was carried out on 50 fatteners (average liveweight 50 kg) divided into 5 groups (5 gilts and 5 barrows per group). The pigs were fed a basal diet supplemented with 0, 0.5, 1, 1.5 and 2% of CLA isomers. Blood samples were collected from fatteners weighing 105 kg. Supplementation of the diet with CLA did not significantly affect lipid parameters in blood plasma. In comparison with gilts, a significantly higher concentration of triglycerides in barrows was found. In CLA-fed pigs, significant changes in the plasma fatty acid profile were noticed. The higher doses of CLA caused a significant increase in CLA and saturated fatty acids (SFA) with a concomitant decrease in monounsaturated (MUFA) and polyunsaturated (PUFA) fatty acids in blood plasma. Moreover, dietary CLA decreased the ∆9 desaturase index in a dose-dependent manner, probably as a result of the inhibitory effect of CLA on stearoyl-CoA activity.Only the lowest dose of CLA (0.5%) increased T ₄ and T₃ concentrations in blood plasma of fatteners by 64 and 27%, respectively. There were no significant changes in rT₃, FT₄ and FT₃ in the plasma of CLA-treated pigs. The results obtained indicate that dietary CLA influenced the fatty acid profile in blood plasma of fatteners without affecting basic lipid parameters. The increase in T₄ and T₃ concentrations in the blood of CLAtreated pigs suggests that low doses of CLA may stimulate the function of the pituitary-thyroid axis and/or deiodinase activity. On the other hand, the absence of changes in FT₄ and FT₃ indicates that CLA treatment does not affect thyroid hormone homeostasis in blood.