CONDITION
Sertoli Cell Tumour
A Sertoli cell tumour is a growth that arises from the supporting cells inside a testicle. These cells ordinarily help produce sperm; when they form a tumour, they often begin secreting hormones — particularly oestrogen — in amounts that can affect the rest of the body. The tumour itself may cause the testicle to enlarge, change shape, or become firm, though in some cases there are no obvious changes to the testicle at all. Owners most commonly notice changes in coat quality, skin darkening, or behavioural shifts in male dogs, sometimes before realising the testicle itself feels different. These tumours are far more likely to occur in a cryptorchid (undescended) testicle, but close to half arise in normally descended testicles, usually in older dogs. In some cases, the first sign is another dog showing unusual interest, or a routine examination identifying an asymmetry. This page explores what signs may appear, what is happening hormonally and within the testicle, how the condition is investigated through examination and imaging, and the approaches available once a tumour is identified. The aim is to help you understand the pattern your dog may be showing and the shape of the conversations that may follow.
Why this matters now
Sertoli cell tumours tend to develop in older intact male dogs, typically appearing after seven or eight years of age, though the pattern varies considerably. Dogs with a retained testicle face a substantially higher risk — cryptorchid testicles are around ten times more likely to develop this type of tumour than those in the scrotum. Certain breeds, including Shetland Sheepdogs, Weimaraners, and some terrier lines, appear more often in case series, though the tumour can arise in any breed.
The tumour often grows slowly over months or years, and hormonal changes may precede any noticeable enlargement of the testicle itself. In many cases, coat changes or skin darkening appear gradually, and owners may assume these are signs of ageing or seasonal shedding until other patterns emerge. Some tumours remain small and cause minimal disruption, whilst others continue to grow and secrete increasing amounts of oestrogen, leading to more pronounced physical and behavioural changes over time.
Signals & patterns
Early signals
Coat thinning or texture change
The hair may become dry, brittle, or sparse, often beginning along the flanks or lower back. Owners sometimes describe a moth-eaten appearance or notice that the coat no longer grows back after clipping.
Darkening or thickening of the skin
Areas of skin, particularly around the lower abdomen, groin, or inner thighs, may become noticeably darker or feel leathery to the touch. This change can be subtle at first and may be mistaken for dirt or bruising.
Asymmetry between the testicles
One testicle may feel firmer, larger, or differently shaped compared to the other. In some cases, the difference is only apparent on careful palpation, and the affected testicle may not feel obviously abnormal in isolation.
Reduced interest in other dogs or mating
Some dogs become less responsive to cues from females in season or show less interest in scent-marking or mounting behaviours. The shift can be gradual and may be attributed to age or temperament.
Attracting attention from other male dogs
Other male dogs may begin sniffing, following, or mounting the affected dog more frequently. This behaviour reflects the changing hormonal scent profile and can occur before any other signs are obvious to the owner.
Later signals
Enlargement of the mammary tissue
The nipples may become more prominent, and the tissue around them can swell or feel thickened. In some cases, this gynecomastia is symmetrical and resembles the appearance seen in female dogs during hormonal cycles.
Widespread hair loss or alopecia
Hair loss may extend beyond the flanks to cover large areas of the body, often with a symmetrical pattern. The skin beneath may remain smooth and soft, or it may become hyperpigmented and prone to secondary irritation.
Swelling or mass in the abdomen
If the tumour arises in a retained testicle located inside the abdomen, the growth may become large enough to create a palpable or visible swelling. Owners may notice this as a firmness or asymmetry when stroking the belly.
Bone marrow suppression signs
In a small proportion of cases, prolonged high oestrogen levels can affect blood cell production, leading to pale gums, weakness, or increased bruising. These signs reflect anaemia or reduced platelet counts and represent a more advanced stage.
Click to read about the biological mechanisms
How this is usually investigated
Investigation typically begins with a physical examination of both testicles, noting size, shape, symmetry, and firmness. If a mass or asymmetry is identified, or if the dog is showing hormonal signs such as coat changes or skin darkening, further tests help clarify whether a tumour is present and what effects it may be having elsewhere in the body. The approach is built outward from what is found on examination, with imaging and blood tests selected according to the individual pattern.
Physical examination
Abdominal ultrasound
Complete blood count
Histopathology
Radiography
Options & trade-offs
Management usually centres on removal of the affected testicle, which addresses both the tumour itself and the source of excess oestrogen. The approach may be adjusted according to the dog's age, general health, and whether the tumour has caused complications such as bone marrow suppression. Different owners and animals find different combinations of timing, surgical approach, and supportive care workable, and the shape of management often becomes clearer once imaging and blood results are available.
Surgical removal of both testicles
Castration removes the tumour-bearing testicle and the contralateral testicle in a single procedure. This eliminates the source of oestrogen, prevents future tumours in the remaining testicle, and allows histopathological examination of the tissue. Hormonal signs such as coat loss and skin darkening often improve over weeks to months following surgery, and bone marrow function typically recovers if it has been suppressed.
Trade-offs: Surgery requires general anaesthesia, which carries higher risk in older dogs or those with concurrent health conditions. Recovery is usually straightforward, though complications such as bleeding, infection, or delayed wound healing can occur. If the testicle is abdominal, the procedure is more invasive than a routine scrotal castration.
Unilateral removal of the affected testicle
In some cases, only the tumour-bearing testicle is removed, leaving the contralateral testicle in place. This approach may be considered when the owner wishes to preserve breeding potential or when there are concerns about the hormonal effects of complete castration, though it leaves the remaining testicle at some risk of future tumour development.
Trade-offs: Removing only one testicle does not eliminate the possibility of a tumour developing in the remaining testicle later, and it does not address any testosterone-related behaviours or conditions. The dog remains intact and capable of reproduction, which may or may not align with the owner's broader plans.
Supportive care for bone marrow suppression
If blood counts show significant anaemia, low platelets, or reduced white cells, management may include blood transfusions, antibiotics to manage infection risk, or careful monitoring whilst waiting for marrow function to recover after surgery. In severe cases, hospitalisation and intensive support may be needed until counts begin to rise.
Trade-offs: Bone marrow suppression is uncommon and usually reverses once the source of oestrogen is removed, but recovery can take weeks. Transfusions and intensive care add cost and complexity, and some dogs remain fragile during the recovery period.
Monitoring without immediate intervention
In dogs with minimal clinical signs, a small or stable tumour, and normal blood counts, some owners and vets choose to monitor the testicle and recheck periodically rather than proceeding to surgery straight away. This approach may suit dogs with significant anaesthetic risk or owners who wish to defer surgery whilst gathering more information.
Trade-offs: Monitoring does not remove the tumour or stop oestrogen secretion, and the tumour may continue to grow or cause progressive hormonal effects over time. It is not a long-term solution for most dogs, and delayed surgery may become more complex if the tumour enlarges or complications develop.
Common misconceptions
"Castration will reverse all the changes caused by the tumour within days."
Hormonal signs such as coat thinning, skin darkening, and behavioural changes often improve following removal of the testicle, but the process typically unfolds over weeks to months rather than days. Hair regrowth in particular can be slow, and some pigmentation changes may persist. Bone marrow recovery, when needed, also takes time.
"A Sertoli cell tumour always causes the testicle to enlarge noticeably."
Many Sertoli cell tumours do cause enlargement or a change in the shape and firmness of the testicle, but some remain small and produce minimal structural change whilst still secreting significant amounts of oestrogen. Hormonal signs such as coat loss or skin darkening can appear before any palpable change in the testicle itself, particularly in cryptorchid cases where the testicle cannot be felt externally.
"If blood tests show bone marrow suppression, surgery must wait until counts recover on their own."
Bone marrow suppression caused by oestrogen from a Sertoli cell tumour tends to persist as long as the tumour remains in place. Surgery to remove the testicle eliminates the source of oestrogen and allows marrow function to recover, though supportive care such as transfusions or antibiotics may be needed in the perioperative period. Waiting without removing the tumour rarely leads to improvement in counts.
Related conditions
Cryptorchidism
Cryptorchidism—retention of one or both testicles within the abdomen or inguinal canal—substantially increases the likelihood of Sertoli cell tumour development, as undescended testicles are exposed to higher temperatures and altered hormonal signalling that may favour tumour formation.
Cushings Disease in Dogs
Dogs with Sertoli cell tumours can develop signs that resemble Cushing's disease—such as symmetrical hair loss, thinning skin, and a pot-bellied appearance—because both conditions involve abnormal hormone secretion, though the hormones and underlying causes differ.
Addisons Disease in Dogs
The hormonal changes seen in Sertoli cell tumour, particularly high oestrogen levels, can occasionally suppress adrenal function or mimic patterns of adrenal insufficiency, though the mechanism and laboratory findings differ from primary Addison's disease.
Adrenal Tumours
Both Sertoli cell tumours and functional adrenal tumours can produce excess hormones that alter coat quality, body shape, and behaviour, and in some cases both types of growth may be considered when investigating a dog with signs of hormone excess and an abdominal mass on imaging.
Dermatophytosis (Ringworm)
The coat changes and hair loss that can accompany Sertoli cell tumour—particularly symmetrical thinning and altered pigmentation—may sometimes be mistaken for dermatophytosis or other primary skin conditions until the underlying hormonal cause is identified.
Once a Sertoli cell tumour has been identified and managed, it can be useful to understand how hormonal changes affect other body systems, including skin, coat, and bone marrow, and what other conditions may share overlapping signs. The broader Metabolic Health pillar explores how hormone imbalances of various kinds influence wellbeing, and may offer additional context for patterns you have observed. If your dog has a cryptorchid testicle, considering the timing and approach to surgery is a conversation that can be had at any routine appointment, even before signs appear.