The experiment was carried out on 96 female broilers, allocated to 8 groups of 12 birds, kept in individual cages. Two wheat- and soyabean meal-based diets containing 150 g/kg of rapeseed expeller cake were formulated, with Ca and P levels of 9 and 7.1 g/kg (diet H) or 7.9 and 5.9 g/kg (diet L), respectively. The diets were unsupplemented or supplemented with phytase (1000 U/kg) and/or acidifier (6 g/kg) and were fed from day 8 to 28 of life. At the end of the experiment the chickens were killed and dissected, the thyroids were weighed, and caecal digesta was collected for short-chain fatty acid (SCFA) analysis. Supplementation of diets with phytase or acidifier significantly increased thyroid weight by 16% (P < 0.01) and 11% (P < 0.05), respectively. The increase in thyroid weight due to phytase supplementation was greater at the lower dietary Ca and P levels, and the greatest when the acidifier was also added to the diet. The thyroid weight did not, however, correlate with either the total SCFA concentration or with the ratio of individual SCFAs in the caecal digesta.