Effects of hypothyroidism and progesterone on mammary tumours induced by 7,12-dimethylbenz(a)anthracene in Sprague-Dawley rats

Hypothyroidism, alone or combined with progesterone, significantly decreased 7,12-dimethylbenz(a)anthracene (DMBA) mammary tumorigenesis relative to controls. However, the decrease was less in the progesterone-treated group, and statistical analysis showed that progesterone enhanced tumorigenesis to the same extent in hypothyroid animals as in the controls. Most tumours in hypothyroid progesterone-treated rats were adenocarcinomata; in the absence of the hormone most tumours were benign. However, the difference between the tumour types in the 2 groups was not statistically significant.