Tumor of the Thymus in Cats


PetMD Editorial

Updated Aug. 24, 2022

Thymoma in Cats


The thymus is a small glandular organ located in front of the heart. Its special function is to serve as a reservoir for the production and maturation of T lymphocytes, white blood cells that make up an important part of the body's immune system. A thymoma is a tumor originating from the epithelium  of the thymus (the layer of tissue covering the thymus). Thymomas are rare tumors in cats and are mainly associated with myasthenia gravis, a severe autoimmune disease which causes certain muscle groups to tire easily.


Symptoms and Types


  • Coughing
  • Increased breathing rate
  • Trouble breathing
  • Cranial caval syndrome -- a side effect of heartworm infestation, which often leads to swelling of the head, neck, or forelimbs
  • Myasthenia gravis, a neuromuscular disease that leads to muscle weakness, enlarged esophagus, and frequent regurgitation




  • Unknown




Your veterinarian will perform a thorough physical exam on your pet, taking into account the background history of symptoms and possible incidents that might have led to this condition. A complete blood profile will be conducted, including a chemical blood profile, a complete blood count, a urinalysis, and an electrolyte panel. 


Thoracic X-rays are standard for breathing related conditions. The resulting images may show a cranial mediastinal mass (a mass between the lungs), pleural effusion (build-up of fluid in the lungs due to aspiration pneumonia) and megaesophagus. 


A blood test for antibodies to acetylcholine receptors (a neurotransmitter causing muscles to contract) should be performed to rule out myasthenia gravis. A tensilon test can also be used to test for myasthenia gravis. 


A fine-needle aspirate of the mass will show mature lymphocytes (white blood cells) and epithelial cells (cells forming in the outside layer of the thymus gland).



Patients should be hospitalized in preparation for surgery to remove the thymoma. These types of tumors are highly invasive and sometimes difficult to remove. Twenty to thirty percent of thymomas are found to be malignant, with metastasis (spread) throughout the chest and/or abdomen.


Living and Management


If the tumor is completely surgically resectable (and has not spread), full remission is generally assured. Your veterinarian will schedule follow-up appointments every three months with you to retake thoracic X-rays of your cat in case the tumor should recur.

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