The effect of dietary fat level and composition on the activities of enzymes involved in thyroid hormone metabolism: thyroid peroxidase (TPO) and hepatic type I deiodinase (DI) were investigated. Male Wistar rats weighing on average 277 g (SEM=4.23 g) received different levels (w/w 5%-LF, 10%-MF, 20%-HF) and types of dietary fat (sunflower oil - group S, rape seed oil - R and palm oil - P) over a three weeks. TPO rose with fat intake in group R and declined in groups S and P. Hepatic DI activity was not affected by dietary fat composition, but was influenced by fat level, decreasing as fat intake increased. The influences of dietary fat level and composition on thyroid physiology are interdependent. TPO and DI activity seem to respond in a differentiated manner to changes in the amount and type of fatty acids consumed.