The objective of the study was to examine the effect of dietary fat source and level on plasma thyroid hormone concentrations. For three weeks, male Wistar rats (n=54) were fed diets containing fats differing in fatty acid (FA) composition: sunflower oil predominantly containing polyunsaturated n-6 FA, group S; rich in monounsaturated FA rape seed oil, group R; and saturated FA palm oil, group P; at three levels (w/w): 5% LF, 10% MF, and 20% HF. Total thyroxine levels were higher in group P than R on the LF and MF diets. The free thyroxine concentration in rats on the LF diet was higher in group R than in S and P; on the MF diet, higher in group R and P than S; and on the HF diet, higher in group P than S. Triiodothyronine levels were influenced by fat composition only in rats fed the HF diet, being lower in group S than P. The results of this study suggest that diets differing with respect to fat type and level might have opposite effects on thyroid hormone values.