CONDITION
Testicular Tumours
Testicular tumours are abnormal growths that develop in one or both testicles of intact male dogs. They arise when cells in the testicular tissue begin to multiply in an uncontrolled way, forming a mass that can vary in size and behaviour. Some testicular tumours grow slowly and remain contained within the testicle, while others can produce hormones that affect behaviour, coat quality, or other parts of the body. Owners often notice a change in the size, shape, or firmness of one testicle, or observe that the two testicles no longer feel symmetrical. In some cases, the first signs are not related to the testicles themselves—these can include hair loss, skin changes, behavioural shifts, or changes in the way other dogs respond to the affected animal. Occasionally, a testicular mass is found during a routine examination when no outward signs have been noticed. This page explores what signs may be observed, what processes lead to different types of testicular tumour, how they are investigated, and what approaches exist once a tumour is identified. The goal is to help orient your understanding of what may be happening and what the shape of the conversation with your veterinary team might look like.
Why this matters now
Testicular tumours are most often identified in older intact male dogs, typically those over seven years of age, though they can occasionally appear earlier. Certain breeds, including boxers, Weimaraners, collies, Shetland sheepdogs, and German shepherds, appear in clinical records more frequently than others. Dogs with retained testicles—those that did not descend fully into the scrotum during development—face a notably higher likelihood of tumour formation in the undescended tissue, sometimes at a younger age.
Many testicular tumours grow slowly over months or years, and some remain clinically silent unless the testicle is examined deliberately. Others begin to produce hormones that create changes in distant parts of the body—skin, coat, behaviour—before the mass itself becomes obvious. The pace and pattern vary considerably between tumour types and individual animals; some masses remain stable for extended periods, while others grow more quickly or begin to affect surrounding tissues or hormone-dependent organs.
Signals & patterns
Early signals
Asymmetry between testicles
One testicle may feel larger, firmer, or differently textured than the other when gently examined. This difference can develop gradually and may be subtle at first, noticed only when comparing the two sides directly.
Altered interest from other dogs
Some dogs with hormone-producing tumours begin to attract unusual interest from other males, or may elicit mounting behaviour, reflecting changes in the chemical signals their body is releasing. Owners sometimes describe this as a shift in how the dog is perceived socially during walks or group interactions.
Thinning or symmetrical hair loss
Hair may become sparse or fall out in a symmetrical pattern along the flanks, abdomen, or lower back, often without obvious itching. This reflects hormonal shifts rather than skin disease, and the coat may lose its usual texture or lustre.
Change in scrotal size or shape
The scrotum itself may appear enlarged, pendulous, or uneven, or one side may seem noticeably heavier than before. These changes can occur independently of pain or discomfort.
Later signals
Feminisation or nipple enlargement
In cases where oestrogen is being overproduced, male dogs may develop enlarged nipples, swelling of breast tissue, or a pendulous sheath. The dog may begin to attract male attention persistently, and behavioural changes such as passivity or reduced territorial behaviour may accompany these physical signs.
Bone marrow suppression signs
Rarely, very high circulating oestrogen levels can suppress the bone marrow, leading to pale gums, lethargy, small bruises under the skin, or nosebleeds. These signs reflect falling numbers of red cells, white cells, or platelets in the bloodstream.
Visible abdominal mass
If a testicle was retained inside the abdomen and has developed a tumour, the mass may eventually become large enough to create a palpable swelling or firmness in the belly, sometimes accompanied by reduced appetite or discomfort when lying down.
Click to read about the biological mechanisms
How this is usually investigated
Options & trade-offs
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