Mammary Gland Enlargement in Cats


PetMD Editorial

Published May 22, 2010

Mammary Gland Hyperplasia in Cats

Mammary gland hyperplasia is a benign condition in which an an excessive amount of tissue grows, resulting in enlarged masses in the mammary glands. This is primarily limited to young, sexually intact, cycling, or pregnant queens, but it can also affect cats of either gender after neutering, and cats of either gender that are on progestogen medication.

Symptoms and Types

  • Enlargement of one or more mammary glands
  • Firm, nonpainful masses in the chest, and abdominal area


  • Secondary to a progesterone influence
  • May develop after neutering; the illness may involve progesterone, growth hormone, or prolactin (a peptide hormone primarily associated with lactation)
  • High progesterone – may be associated with false pregnancy in a queen that has been induced to ovulate but has remained nonpregnant for 40–50 days after ovulation induction, or in pregnant queens throughout the gestation period
  • Associated with administration of prescription progestogen


Your veterinarian will need to differentiate between several possibilities to arrive at a reliable diagnosis. Fluid will be expressed from the mammary gland for laboratory analysis, and a biopsy of the tissue may also be analyzed to determine the exact cause of the excessive tissue growth, and whether the growth is in fact, of a benign or malignant (cancerous) nature. Mastitis (infection of the mammary glands) can usually be ruled out based on the absence of symptoms, such as painful glands and fever, but the presence or absence of bacteria in the expressed fluid will definitively rule out infection.


If the enlargement is due to high levels of progesterone, the mass will diminish as the levels fall at the end of the false pregnancy or gestation. To lower progesterone levels permanently, a hysterectomy may be considered if fertility is not an issue. If the enlargement is related to the use of prescription progestogens, the mass will diminish when the medication is withdrawn.

Enlargement that occurs after neutering will resolve itself spontaneously. If your cat is uncomfortable, your veterinarian may prescribe progesterone receptor blockers, or prolactin inhibitors (a peptide hormone that is primarily associated with lactation).

Living and Management

The likelihood of recurrence in cats that are left intact is unknown, as is any correlation with other abnormal conditions of the reproductive tract.

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