Why Is There A Lump Under My Dog’s Armpit?

lump under dogs armpit

Lumps on a dog’s body are a common occurrence and can be caused by a wide range of things. 

The implications of these lumps also vary, with some lumps being totally harmless and some being cancerous and potentially fatal.

If you find a lump under your dog’s armpit, it’s probably lipoma, a noncancerous growth of fatty tissue under the skin.

One of the most common places where lipoma grows is under the armpit.

Here is what you will learn from the article:

  1. How can a lump under my dog’s armpit be treated?
  2. Is it bad if my dog has a lump?
  3. What are the main causes of lumps in a dog’s body?
  4. Where are the main places (on the body) that lumps can be found?
  5. What are other symptoms of cancer apart from lumps?
  6. Why are some lumps hard and others soft?
  7. Why do some lumps move and others don’t?
  8. Do cancerous lumps hurt?
  9. How quickly can cancerous lumps grow?
  10. When should I worry about my dogs lumps?

How can a lump under my dog’s armpit be treated?

If the lump under your dog’s armpit is lipoma, the way to treat it is by performing liposuction or removing it surgically.

Lipomas aren’t dangerous so sometimes the vet won’t recommend surgery and instead suggest monitoring.

This just means that you have to regularly check the lump for any change of color, size, and firmness, and go to the vet for a regular check.

Is it bad if my dog has a lump?

In general, a lump on a dog’s body can be divided into 2 categories:

  • Benign: is a noncancerous lump, meaning it won’t spread to other tissues.
  • Malignant: is a cancerous lump. The growth will spread to other parts of the body.

Most of the time, benign lumps are fine and they’ll heal quickly with simple treatments.

Malignant lumps on the other hand should be a cause for worry.

At times a benign lump too can turn into a cancerous one, but this is rarely the case. Read on to be able to differentiate the two.

What are the main causes of lumps in a dog’s body?

As stated above, lumps in dogs can be either benign or malignant. Here’s a list of the most common kinds of lumps in a dog’s body complete with each of the causes.

  1. Lipoma

The most common lump in dogs is lipoma. Lipoma is a noncancerous growth of fatty tissue that grows under the skin, typically in the limb, armpit, and groin. It is colorless and grows slowly. Lipomas might turn into liposarcomas – which is cancerous – but the chance is very small.

Lipoma is not dangerous or life-threatening, sometimes it can even go unnoticed.

But a lipoma may cause discomfort to your dog and hinder some of their movement.

At most, a growing lipoma might affect a dog’s blood flow inducing pain.

The lump can shrink with weight loss or by performing liposuction, but the only way to get rid of lipoma is by surgically removing it.

If surgery isn’t performed, the vet will monitor the lump – noting the size, shape, and firmness – and only remove it if it becomes really bothersome to the dog.

Even after being surgically removed, lipoma may show up again in another place. Lipoma removal surgery is usually followed by radiation therapy to prevent regrowth.

Cause of lipoma in dogs:

Until now, health experts have found no exact reason for lipoma. It’s believed to be caused by overall poor health and environment such as inadequate diet and overexposure to toxins.

Overweight adult dogs are more likely to get lipoma. Dog breeds that have a higher risk of lipoma are Labrador retrievers, Golden retrievers, Doberman pinschers, and Cocker spaniels.

  1. Sebaceous cyst

Sebaceous cyst is a tumor filled with sebum that grows around sebaceous glands.

Just like lipoma, sebaceous cyst is noncancerous, but very on rare occasions it can become malignant.

Oil glands are located in the skin all over the body, so sebaceous cysts can grow in anywhere, with the most common locations being the upper limb, neck, and chest.

Compared to lipoma, sebaceous cysts are really small, looking like a smooth white little bump under a dog’s skin.

The lump is usually slow-growing and painless, and sometimes the surface ulcerate.

A sebaceous cyst is harmless, but if the dog scratches the cyst, germs and yeast on the skin may contaminate the area causing infection.

An infected cyst’s surrounding area may become red, swollen, or odorous.

The cyst might also run out of room to grow and rupture, causing discharge or even bleeding.

Any of these symptoms indicate that your dog requires medical attention. Your dog will be treated with topical treatments and antibiotics.

Cause of sebaceous cyst in dogs:

Sebaceous cyst is caused by blockage of the follicle, affecting the oil glands. Some factors that may increase the risk of developing sebaceous cyst are skin injury, accumulation of scar tissue, overexposure to UV, and hair follicle inactivity in hairless breeds.

Dog breeds more susceptible to sebaceous cysts are Yorkshire Terriers, Shih Tzus, Boxers, Basset Hounds, Doberman Pinschers, and hairless breeds.

  1. Abscess

Abscess is the condition of an infected wound that swells into a lump filled with pus. Abscesses are usually found anywhere below the skin surface, on the root of a tooth, and anal sac, but they can also develop on internal organs. External abscesses (on the skin or tooth root) aren’t dangerous, but internal abscesses can be fatal. Abscesses aren’t tumors and therefore noncancerous.

The lump is red, soft and squishy, sometimes firm, and warmer than the surrounding skin. It’s filled with pus and would ooze from the abscess emitting a foul odor. An abscess might cause other symptoms like lethargy and appetite loss.

To get rid of an abscess, vets usually recommend opening and draining the lump. If the source of infection is still intact – for example if there’s a foreign material stuck under the skin –, the vet will remove it. The site of infection will be cleaned, including trimming the hair around the area.

Cause of abscesses in dogs:

Abscesses are caused by a bacterial infection within a wound. Skin abscesses are induced by trauma on the skin, including bite wounds, insect stings, splinters and even stepping on something big or sharp. These events allow the bacteria to be trapped inside the body.

  1. Melanoma

Melanoma is a cancerous growth on the skin affecting pigmented cells called melanocytes. It can grow exponentially bigger and spread to other parts of the body. Melanomas appear as pink pigmented masses that grow bigger, bleed, and ulcerate.

It’s often found in the oral cavity, nailbed, foot pad, and areas where the skin meets the mucous membranes such as lips, vulva, and anal. Melanomas on the skin are usually benign and curable, but if it occurs in areas where the skin meets the mucous membranes the tumor is often aggressive and spreads quickly.

Melanoma is treated with surgical resection, chemotherapy, and radiation therapy. There’s also canine melanoma vaccine that can help fend off abnormal tumor cells.

Cause of melanoma in dogs:

Sadly, the cause of melanoma is still unknown. Melanoma is found to be more prevalent in Golden Retrievers, Cocker Spaniels, Poodles, and Chow Chows.

  1. Mastocytoma (mast cell tumor)

Mast cell tumor is the most common type of skin cancer in dogs located in mast cells (cells that play a role in allergy response). Mast cell tumors on the skin can occur in any part of the body, most commonly on the trunk and perineum.

This tumor usually shows up as a red, swollen, and ulcerated lump under the skin. It usually appears suddenly and grows quickly.

MCT is one of the most treatable cancers, especially in the lower-grade stage. Same as melanoma, treatment will be done by surgery, chemotherapy, and radiation.

Cause of mast cell tumor in dogs:

As with most cancers, the cause of mast cell tumor is unknown, seemingly caused by a complex mix of risk factors from environmental to genetic. MCT is more prevalent in Labrador Retrievers, Boxers, Boston Terriers, and Bull Terriers.

Other cancerous, less common lumps on a dog’s body are soft tissue sarcoma, squamous cell carcinoma, and mammary carcinoma.

Where are the main places (on the body) that lumps can be found?

  • Limb, armpit, or groin (lipoma)
  • Upper limb, neck, and chest (sebaceous cyst)
  • Lips, vulva, anal (melanoma)
  • Trunk and perineum (mast cell tumor)
  • Hairless parts of the body (squamous cell carcinoma)
  • Mammary area and armpits (mammary carcinoma)

What are other symptoms of cancer apart from lumps?

  • Lameness (inability to walk or use limbs properly)
  • Pigmented sores
  • Swollen lymph nodes
  • Wounds that won’t heal
  • Gastrointestinal problems
  • Sudden collapse
  • Labored breathing
  • Unexplained weight loss
  • Lethargy

Why are some lumps hard and others soft?

Lumps can be caused by a wide range of reasons, and depending on the cause, the lump will feel different.

For instance, if a lump feels dense and hard, this is usually a sign of cancerous growth. On the other hand, a lump that feels soft is benign, typically lipoma.

This is due to cancer cells’ ability to change the structure of the tissue. Cancer cells tend to make the tissue around a tumor stiffer and harder.

Why do some lumps move and others don’t?

Similarly, there are also lumps that move when touched and some that are immovable. Immovable lumps can be a sign of cancer.

Do cancerous lumps hurt?

Despite being a sign of terminal illness, cancerous lumps usually don’t hurt. 

How quickly can cancerous lumps grow?

Another way to distinguish a cancerous lump from a milder case of lumps is by seeing how fast it grows. Most of the time, cancerous lumps grow very fast

When should I worry about my dogs lumps?

To sum it up, these are signs of cancerous lumps on a dog:

  • The lump grows very fast,
  • feels hard and stiff (immovable),
  • swells,
  • doesn’t hurt,
  • has an opening
  • has pus.

If the lump on your dog’s body has one or more of the characteristics above, go to the vet immediately to get your pup checked.

James Grayston

My name is James and I love dogs. have owned four Golden Retrievers in the past 15 years. Currently I own two "Goldies"- a five year old and a seven month old. The photo shows me with our youngest when she was about 7 weeks old!