One would expect their female dog to produce milk after she has had puppies. Female dogs can produce milk that is secreted by their nipples, as any female mammal can. But dogs may even produce milk—or what looks like milk—through their nipples when they are not pregnant and have not recently given birth. Causes include:

Most of the time, milk production will be either a normal and healthy physical function, or at least not dangerous. However, there can be sinister underlying causes, so a milk-producing dog who is not recently or currently pregnant should be examined by a veterinarian.

Female dogs produce milk, or something with the appearance of milk, for two sets of reasons. Most of the time it is related to real or perceived pregnancy, but it can also be related to various illnesses. 

Female dogs spontaneously produce milk when they have given birth to puppies. This is a normal part of biological parenthood for any female mammal. 

Female dogs may also produce milk when experiencing a false pregnancy or when another creature attempts to nurse from her. A false pregnancy is a condition where a female dog exhibits various symptoms of pregnancy, such as producing milk and building a nest for her puppies, but is not pregnant. Female dogs can also produce milk when another creature, such as a puppy or kitten, sucks and kneads her mammary area, even though she may not be pregnant or have recently given birth. This would have been essential when dogs lived in the wild in the case of orphaned pups.

Hypothyroidism occurs when the circulating level of thyroid hormone (T4) is too low. This causes the dog’s metabolism to slow down, resulting in symptoms that are typically associated with old age, such as lethargy, weight gain, loss of muscle coordination, and many more. Hypothyroidism usually occurs in medium to large dogs between 4 and 10 years old, and can be caused by lymphocytic thyroiditis, thyroid cancer, improper level of iodine, being born with it (congenital hypothyroidism), or for no clear reason.

Mammary gland tumors are growths in one or more of a female dog’s eight to ten teats. They may or may not be cancerous. Hormones are a clear factor in mammary cancer, as only half of one percent of dogs spayed before their first heat ever develop the disease. Toxins, such as pesticides and household chemicals, may be a risk factor, but there are no clear and certain non-hormonal causes of mammary gland tumors. In the case of a mammary gland tumor, the white substance produced may look like milk, but it is actually pus.

If your dog is pregnant or has recently given birth, she should be producing milk. Leave her to it. Neither are a false pregnancy or spontaneous nursing problematic. They are interesting phenomena, but they do not typically present a health problem. Discomfort or engorgement of the mammary glands may occur in any of these cases; these may be eased with alternating warm and cold compresses. However, hypothyroidism or mammary gland tumors require immediate medical attention.

Hypothyroidism is the less serious of the two conditions. It cannot be cured, but it can be treated, and a dog with medicated hypothyroidism can live a normal and healthy life. Your dog’s veterinarian will give your dog thyroxine, an oral hormonal replacement, which she will take for the rest of her life. The dosage may need to be adjusted as your dog’s weight and drug tolerance fluctuates. Mammary gland tumors will need to be surgically removed, possibly followed by chemotherapy or, less likely, radiation. Following treatment, the owner should take care to regularly check the dog’s mammary glands for a recurrence of tumors.

There is no need to prevent a normally lactating female dog from producing milk, even if it is experiencing a false pregnancy or nursing without being pregnant. Hypothyroidism is typically not preventable; however, mammary gland tumors are almost entirely preventable by spaying your female dog before her first heat.

A normally lactating dog does not require any medical treatment, nor does a dog experiencing a false pregnancy or spontaneous milk production for another nursing animal. The average cost of treating hypothyroidism is $1300, while it costs an average of $5500 to treat mammary gland tumors.