Testicular Tumor (Leydig Cell) in Dogs

3 min read

Tumor of the Interstitial Cell of the Testicle

The Leydig cell tumor (LCT) is a rare, and typically benign (non-spreading) tumor made up from the cells that release the testosterone hormone in the connective tissue of the testicles. This type of tumor may occur singularly, or in multiples.  The tumor mass is located in the testis, causing soft swelling of the affected testis. It measures about 1-2 cm in diameter and is spherical in shape. It is classified as a sex-cord stromal tumor, meaning that the tumor issues from the connective tissues of the sex-cords of the testis. This condition affects older male dogs.

Symptoms and Types

  • One or more round masses (1-2 cm in diameter) in a testicle
  • There are generally no symptoms with this type of tumor, unless it is actually a Sertoli cell tumor (cells which help nourish spermatids as they transform into spermatozoa in the testicles)
  • Signs of a Sertoli cell tumor:
    • Feminization (from secretion of estrogen)
    • Bone marrow underdevelopment



The cause of LCT is unknown, but a retained testicle (usually in the abdomen) may predispose an animal to Leydig cell tumor formation.


Your veterinarian will perform a complete physical exam, palpating (examination by touch) your dog’s testicles to examine the size, location, and consistency of the tumor. You will need to provide a thorough history of your dog’s health, with a description of symptoms, if any, and their onset. Usually a blood chemical profile, complete blood count, and urinalysis will return as normal, but there may be various reductions of cells in the circulating blood if there is an excess of estrogen. Blood serum should also be tested for estradiol, an estrogenic hormone, and testosterone concentrations. Usually estradiol levels will be high, while testosterone levels will be low. Your veterinarian may also take a fine needle sample of fluid (aspirate) from the tumor to check for abnormalities in the cells, by use of a cytological (microscopic) examination

Tumors smaller than 3 cm in diameter will appear black on ultrasound imaging. However, tumors greater than 5 cm have a black and white blotched appearance on ultrasound.

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