Hormones and mammary tumour disease in the female dog: an update.

In both toxicity studies and epidemiologic studies in veterinary medicine it has been demonstrated that ovarian steroids as well as many synthetic derivatives may enhance mammary tumour formation in the dog. Abnormalities in pituitary function, in particular of growth hormone secretion, have been assumed to take part in this process. In this paper the pathogenetic role that endogenous and exogenous hormones may have in canine mammary tumourigenesis is reviewed. The available evidence suggests that steroid hormones act at an early stage in the development of tumours by stimulating the proliferation of normal epithelium. This leads to an increase in the number of susceptible cells. A growth-stimulatory action may further be exerted upon cells that have undergone partial malignant transformation, but possibly to a lesser extent upon fully malignant cells at a late stage of tumour development. In advanced mammary cancers steroid receptor presence is infrequent, which may indicate a more autonomous pattern of growth. No support has been found for the earlier assumption that growth hormone overproduction is an important factor in canine mammary tumourgenesis. The role of prolactin and thyroid hormones in this process remains uncertain.