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Effect of 3,5,3'-Triiodo-L-thyronine on the Incidence and Growth Kinetics of Spontaneous Mammary Tumors in C3H/HeN Mice¹

Uniformed Services University of the Health Sciences, Department of Medicine, Bethesda, Maryland 20014

The present study compares the incidence and growth kinetics of spontaneous mammary tumors in 3,5,3'-triiodo-L-thyronine (T₃)-treated (0.7 µg/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 group. 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.

¹ This research was supported by Grants CO8309 and CO8313 by The Uniformed Services University of the Health Sciences, Bethesda, MD.

Received 4/ 6/81. Accepted 9/14/81.