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
Investigation typically begins with a physical examination of both testicles, comparing size, shape, firmness, and symmetry. The veterinary team will take a history that includes any changes in behaviour, coat, or skin, as well as the age at which neutering might have been considered and whether either testicle failed to descend. As the picture develops, targeted tests help clarify the type of mass present, whether hormones are being produced, and whether there is any spread beyond the testicle itself.
Physical examination
Ultrasonography
Blood biochemistry and haematology
Thoracic and abdominal radiography
Histopathology
Options & trade-offs
Management is shaped by the type of tumour, whether it is producing hormones, the age and overall health of the dog, and the practicalities that matter to the household. Most testicular tumours are addressed through surgical removal of both testicles, which simultaneously removes the mass, eliminates hormone production, and prevents future tumours in the remaining testicle. In some situations, observation or additional treatments may form part of the overall approach, and the combination chosen will vary between animals and their circumstances.
Surgical castration
Removal of both testicles eliminates the source of the tumour and any hormone production, and allows the tissue to be examined under the microscope. The procedure is typically performed under general anaesthesia, and recovery is usually straightforward in otherwise healthy dogs. In cases where one testicle is retained in the abdomen, the surgery involves an abdominal incision rather than a scrotal approach.
Trade-offs: Castration is a permanent change that affects hormone balance, metabolism, and behaviour, and some owners prefer to delay neutering until a tumour makes it necessary. Dogs with significant bone marrow suppression from oestrogen toxicity may require stabilisation before anaesthesia is considered safe, and very elderly or frail animals may face higher anaesthetic risk.
Observation without intervention
In some older dogs with small, slow-growing masses and no hormone-related signs, owners and veterinary teams may choose to monitor the testicle over time rather than pursue surgery. This can involve regular physical examinations and, occasionally, repeat imaging to assess whether the mass is changing. Observation may be considered when anaesthetic risk is high or when the tumour type is thought likely to remain benign.
Trade-offs: Observation leaves the tumour in place, so there is no tissue diagnosis and no removal of hormone-secreting cells if they are present. The mass may grow, begin to produce hormones, or—rarely—spread, and the window for safer surgery may narrow as the dog ages or if complications develop.
Stabilisation before surgery
Dogs with severe anaemia or low platelet counts caused by oestrogen toxicity may need supportive care before castration can proceed safely. This can include blood transfusions, monitoring of blood cell counts, and time for the bone marrow to begin recovering once the source of oestrogen is removed or suppressed. Surgery is then performed once the risk of bleeding or poor oxygen delivery has lessened.
Trade-offs: Stabilisation adds time and cost, and recovery of bone marrow function is not always predictable. In some cases, the marrow damage is severe enough that full recovery does not occur even after the tumour is removed, and ongoing monitoring or further supportive care may be needed.
Chemotherapy or radiation therapy
For the rare testicular tumours that have spread to lymph nodes or distant sites, chemotherapy may be discussed as a way to slow further progression or manage residual disease after surgery. Radiation therapy is occasionally considered for tumours that cannot be fully excised or for local control of malignant seminomas. Both approaches require referral to a specialist centre and involve multiple visits over weeks or months.
Trade-offs: Chemotherapy and radiation carry their own side effects, costs, and time commitments, and the evidence base for their use in metastatic testicular tumours in dogs is limited. Most testicular tumours in dogs are cured by castration alone, so these treatments are relevant only in a small subset of cases where spread has been confirmed or strongly suspected.
Common misconceptions
"If both testicles feel normal in size, there cannot be a tumour present."
Some testicular tumours grow within the substance of the testicle without enlarging it noticeably, and others develop in a retained testicle that is not palpable at all. Hormone-related signs such as hair loss or behavioural change can be the first clue, even when both descended testicles feel unremarkable on examination.
"All testicular tumours spread and are life-threatening."
The majority of testicular tumours in dogs are benign or have very low metastatic potential, and many are cured by castration alone. Interstitial cell tumours and most seminomas remain localised, and even Sertoli cell tumours, which can cause serious hormonal effects, rarely spread if removed before extensive bone marrow damage occurs.
"Neutering a young dog prevents all testicular tumours, so there is no benefit to removing the testicles later in life once a tumour is found."
Neutering before puberty does prevent testicular tumours from developing, but once a tumour is present, castration serves a different purpose: it removes the existing mass, stops any hormone production, and prevents a second tumour from forming in the other testicle. The decision about timing depends on the individual dog's history and current health, not on whether earlier neutering might have been protective.
Related conditions
Cryptorchidism
Cryptorchidism—the retention of one or both testicles in the abdomen or inguinal region—is a developmental difference that substantially increases the risk of testicular tumour formation, particularly Sertoli cell tumours. Testicles that do not descend tend to experience abnormal temperatures and hormonal environments, which may contribute to the later development of neoplastic change.
Sertoli Cell Tumour
Sertoli cell tumour is one of the three main types of testicular tumour in dogs, alongside interstitial cell tumours and seminomas. It arises from the supporting cells within the testicle and often produces oestrogen, which can lead to signs such as hair loss, skin changes, and attraction of other male dogs—patterns that may prompt investigation of a testicular mass.
Prostatic Hyperplasia
Prostatic hyperplasia and testicular tumours both arise in the context of hormonal influence in intact male dogs, and the two can occasionally coexist. Some testicular tumours, particularly those that secrete oestrogen, may influence prostate tissue indirectly, though prostatic enlargement more often reflects normal ageing processes in response to testosterone.
Adrenal Tumours
Adrenal tumours and testicular tumours are both endocrine neoplasms that can produce hormones capable of altering behaviour, coat quality, or metabolic balance. While they arise in different organs, both may present with similar systemic signs when hormonally active, and both benefit from a structured approach to imaging and hormonal assessment.
Prostate Disease
Prostate disease and testicular tumours share an anatomical and hormonal context in intact male dogs, and clinicians often examine both structures when investigating signs such as difficulty urinating, straining to defecate, or changes in hindlimb gait. Hormonal changes from certain testicular tumours can occasionally influence prostatic tissue, though the two conditions more often exist independently.
Understanding what is happening in the testicle can open related questions about how hormone changes affect skin, coat, and behaviour, and what other age-related conditions may appear alongside testicular masses. The broader context of ageing and healthspan, including how different body systems interact as dogs grow older, may be useful to explore at a time that suits your household. Some owners find it helpful to think through what they would want to know before a surgical procedure is scheduled, or to clarify what signs might indicate a change in the mass over time if observation is part of the plan.