If you are pregnant then you may see milky discharge from nipples/breasts before giving birth. But if you are Not then there is reason to be concerned about.
Such milk secretions happen due to imbalance in prolactin hormone that is produced by pituitary gland, which in turn is controlled by thyroid gland. In medical terminology this condition is called Galactorrhea or Hyperprolactinemia depending on various other symptoms. Prolactinoma is often associated with tumor ( enlargement of Pituitary gland ) that results in increased secretion of prolactin. However, such tumors are non-cancerous and won’t spread to other tissues in brain. Also, these rarely need surgery or radiation treatment. Usually these tumors can be shrinked by taking medicines like bromocryptine.