Dietary thymol, and its isomer, carvacrol, were evaluated as alternatives to antibiotic feed additives in female broiler chickens. In addition, the alleged hypocholesterolemic effect of carvacrol and thymol were tested when the chickens were fed cholesterol-free or cholesterol-containing diets. The experiment had a 2 × 3 factorial arrangement of treatments with diets containing two levels of cholesterol (0 or 1%) and without feed additive or with 200 ppm thymol or carvacrol.

Dietary carvacrol lowered feed intake and weight gain but also lowered the feed-to-gain ratio. Dietary thymol, an isomer of carvacrol, did not affect growth performance. Dietary cholesterol significantly increased plasma and liver cholesterol concentrations. Carvacrol lowered plasma triglyceride concentrations but did not affect plasma cholesterol. It is concluded that thymol and its isomer, carvacrol, have different effects on growth performance and triglyceride metabolism in broiler chickens. The two compounds did not have hypocholesterolemic activity, irrespective of whether the diet was cholesterol free or cholesterol rich.