Effect of 3,5,3'-triiodo-L-thyronine on the incidence and growth kinetics of spontaneous mammary tumors in C3H/HeN mice.

The present study compares the incidence and growth kinetics of spontaneous mammary tumors in 3,5,3'-triiodo-L-thyronine (T3)-treated (0.7 microgram/day, i.p.) C3H/HeN retired breeder mice with tumors occurring spontaneously in this strain (0.15 M NaCl, i.p.). T3 treatment markedly increased the rate of tumor incidence but had no effect on tumor growth rate as measured by tumor doubling time. A comparison of tumor labeling index and mitotic index between the two groups showed no difference. A histological comparison of the mammary glands in the two groups showed no significant difference in number of hyperplastic alveolar nodules. However, ductal-alveolar development was more pronounced in the 0.15 M NaCl-treated control group; consistent with this observation was the finding of lower levels of serum prolactin in the T3-treated groups. Since mammary tumorigenesis was increased in T3-treated mice in spite of lower levels of serum prolactin, a well-known promoter of spontaneous mammary tumors in mice, a direct role for T3 in the process of tumor formation is suggested.