Sertoli Cell Tumors and Feminizing Skin Changes in Male Dogs

By Emiel Maddens · Reviewed in consultation with licensed veterinary professionals · Updated August 2026 · 9 min read

What Is a Sertoli Cell Tumor in Dogs?

A Sertoli cell tumor is a testicular neoplasm arising from the Sertoli cells (the nurse cells that support developing sperm inside the seminiferous tubules), and it is the testicular tumor most likely to secrete hormones that visibly change a male dog's skin and coat. Along with seminomas and interstitial (Leydig) cell tumors, it is one of the three common canine testicular tumors, and it accounts for roughly a quarter to a third of all testicular tumors in dogs.

What makes this tumor a dermatology problem rather than purely a surgical one is its endocrine activity. A subset of Sertoli cell tumors overproduce estrogen and inhibin, and that hormonal shift is what drives the skin signs owners actually notice. Estimates from referral caseloads place the proportion of Sertoli cell tumors causing clinical hyperestrogenism (excess circulating estrogen) at approximately 25% to 40%, with feminizing signs far more common in tumors located in a retained abdominal testicle.

Affected dogs are usually middle-aged to senior, with a median age around 10 years. Boxers, Shetland Sheepdogs, Weimaraners, German Shepherds, and Collies are over-represented in published case series, though any intact male can be affected. The single strongest risk factor is not breed but cryptorchidism: a testicle that never descended into the scrotum sits at core body temperature, and that thermal environment is strongly associated with neoplastic transformation.

Because the skin changes appear gradually and are not itchy, many owners assume they are watching normal aging. That delay matters, since the same hormone driving the hair loss is simultaneously suppressing the bone marrow.

What Causes the Feminizing Skin Changes?

The skin changes are caused by estrogen, not by the tumor mass itself. Neoplastic Sertoli cells retain the hormone-producing machinery of normal Sertoli cells but lose normal feedback control, so they secrete estradiol and inhibin continuously and independently of the pituitary signals that would normally shut them down.

Estrogen acts directly on the hair follicle. It prolongs telogen, the resting phase of the hair cycle, and blocks follicles from re-entering anagen (the growing phase). Because the hair cycle in dogs is regionally synchronized, the result is bilaterally symmetrical alopecia (hair loss that mirrors on both sides of the body) beginning over the flanks, perineum, and ventral abdomen. There is no inflammation, so there is no itch, which is a key diagnostic clue.

Estrogen also stimulates melanocytes, producing the diffuse hyperpigmentation that gives affected skin a gray-brown, velvety appearance. In the male dog it simultaneously suppresses the hypothalamic-pituitary-gonadal axis, so the contralateral testicle atrophies, the prepuce becomes pendulous, and the mammary glands enlarge. Some dogs become attractive to other male dogs, a behavioral sign owners often report before they report the hair loss.

The most dangerous consequence is hematologic. Sustained estrogen exposure is myelotoxic in dogs, suppressing the bone marrow and producing a progressive pancytopenia (a fall in red cells, white cells, and platelets simultaneously). Published case series report estrogen-induced myelosuppression in a minority of hyperestrogenic dogs, but when it develops the mortality rate is high, with some reports exceeding 50% despite castration and supportive care. This is why an apparently cosmetic coat change deserves prompt workup.

How Do Veterinarians Diagnose a Sertoli Cell Tumor?

Diagnosis starts with a careful physical examination of both testicles, or a search for the missing one. A veterinarian palpates for asymmetry, since the tumor-bearing testicle is typically enlarged and firm while the opposite testicle is small and soft from hormonal suppression. In cryptorchid dogs the tumor may sit in the inguinal canal or deep in the abdomen, where it is found on abdominal ultrasound rather than by palpation.

Abdominal and scrotal ultrasound is the imaging workhorse. It identifies retained testicles, characterizes the internal architecture of an enlarged testicle, and screens sublumbar lymph nodes for the metastasis that occurs in a small percentage of cases, generally cited at under 15% for Sertoli cell tumors overall.

A complete blood count is not optional. Because estrogen toxicity is the life-threatening complication, every dog with suspected hyperestrogenism should have a CBC checked before and after surgery, with follow-up counts for several weeks. Early changes include thrombocytopenia and neutropenia; anemia appears later because canine red cells have a long lifespan of roughly 110 days.

Skin biopsy is rarely needed to make the diagnosis but is useful when the presentation is atypical. Histopathology of estrogen-driven alopecia shows follicular atrophy with predominance of telogen follicles, epidermal melanosis, and no meaningful inflammatory infiltrate. Definitive confirmation comes from histopathology of the removed testicle. If you are trying to sort a symmetrical hair loss pattern into endocrine categories before your appointment, the Dog Skin Condition Checker can help you organize what you are seeing, and our comparison of Cushing's vs Hypothyroidism skin changes covers the two most common endocrine mimics.

Signs and Symptoms to Watch For

The clinical picture is distinctive once you know what to look for: symmetrical hair loss without itch, darkened skin, and physical feminization in an intact male dog. The most specific single sign is linear preputial dermatosis, a hyperpigmented stripe running along the ventral midline from the prepuce toward the scrotum. It is reported in a substantial share of hyperestrogenic dogs and is considered close to pathognomonic when combined with testicular asymmetry.

Other signs include a pendulous prepuce, gynecomastia (mammary enlargement), atrophy of the unaffected testicle, prostatic changes such as squamous metaplasia, standing in a female urination posture, and attracting other male dogs. Coat quality often changes before frank hair loss, with the guard hairs becoming dull, dry, and easily epilated.

Because estrogen strips the skin of its normal barrier quality and leaves it warm, thickened, and greasy in intertriginous areas, secondary Malassezia (yeast) overgrowth is a frequent complication in the groin, axillae, and preputial fold. That secondary infection is usually what finally turns a silent problem into an itchy, malodorous one.

Treatment Options and Skin Recovery

The definitive treatment is bilateral castration with removal of the tumor-bearing testicle, and in most non-metastatic cases it is curative. Both testicles are removed even when only one is abnormal, because the contralateral testicle carries its own tumor risk and contributes nothing while suppressed. In cryptorchid dogs, an abdominal approach is used to locate the retained testicle, and the surgeon inspects sublumbar lymph nodes at the same time.

Once the estrogen source is gone, circulating estradiol falls within days, follicles re-enter anagen, and coat regrowth typically becomes visible over 2 to 3 months, with full recovery of coat density sometimes taking six months. Hyperpigmentation fades more slowly than the hair returns. Dogs that were pancytopenic before surgery need weeks of monitoring, transfusion support, and in some cases bone marrow stimulants, and their prognosis depends on how severely the marrow was suppressed at presentation.

The dermatologic work does not end with surgery. Skin that has been estrogen-affected for months is often colonized with Malassezia pachydermatis in the preputial fold, groin, and axillae, and that overgrowth persists after castration until it is addressed directly. Topical antifungal and antiseptic therapy is the standard first-line approach for localized Malassezia dermatitis. Yeast Dermatitis Spray combines ketoconazole, an azole antifungal that blocks fungal ergosterol synthesis and destabilizes the yeast cell membrane, with chlorhexidine, a cationic biguanide that disrupts bacterial and yeast cell membranes, which is why the pairing is used for the mixed yeast and bacterial overgrowth typical of these intertriginous sites. It is applied to affected areas rather than as a whole-body treatment, which suits the focal distribution seen here.

For background on the underlying condition and on other hormone-driven skin disease, see our guides to yeast infections in dogs, sex hormone dermatosis in dogs, and castration responsive dermatosis.

Prevention and Long-Term Management

Prevention is straightforward and highly effective: a neutered dog cannot develop a testicular tumor. The clinically important version of that advice concerns cryptorchid dogs, where the retained testicle carries a risk of neoplastic transformation reported at up to 13 times that of a normally descended testicle. Veterinary consensus is that retained testicles should be removed rather than monitored, and that cryptorchid dogs should not be used for breeding, since the trait is heritable.

For owners who keep their dogs intact by choice, the practical management is surveillance. Palpate both testicles monthly and note any asymmetry in size or firmness. Watch for the early feminizing signs, which almost always precede dramatic hair loss: a pendulous prepuce, a darkening midline stripe, mammary enlargement, and unusual interest from other male dogs. Any intact male over eight years old benefits from testicular palpation at every wellness visit.

After successful castration, long-term management is mostly dermatologic maintenance while the coat recovers. Keep skin folds clean and dry, since the moisture and warmth of intertriginous skin favor yeast recolonization even after hormone levels normalize. Recheck bloodwork on the schedule your veterinarian sets, since marrow recovery is the variable that determines outcome in the small subset of dogs that were myelosuppressed.

Recurrence after complete removal is uncommon, and metastasis is rare, so most dogs return to a normal coat and a normal lifespan. The prognosis worsens sharply only in two scenarios: metastatic disease at diagnosis, and severe estrogen-induced bone marrow failure. Both are arguments for acting on symmetrical hair loss in an intact male dog rather than waiting to see whether it resolves on its own.

Frequently Asked Questions

What are the first signs of a Sertoli cell tumor in a dog?

The earliest signs are usually subtle feminization rather than hair loss: a pendulous prepuce, slight mammary enlargement, and a darkening stripe on the midline in front of the scrotum. Symmetrical, non-itchy hair loss over the flanks and perineum follows. Roughly 25% to 40% of Sertoli cell tumors produce enough estrogen to cause these visible changes.

Will my dog's hair grow back after the tumor is removed?

In most cases yes. Once the estrogen source is removed by castration, hair follicles re-enter the growth phase and visible regrowth usually begins within 2 to 3 months, with full coat density sometimes taking up to six months. Skin darkening fades more slowly than the hair returns.

Are Sertoli cell tumors in dogs cancerous or benign?

Most behave benignly. Metastasis is reported in under 15% of cases, typically to the sublumbar lymph nodes, and complete castration is curative for the majority of dogs. The greater danger is not spread but estrogen toxicity, which can suppress the bone marrow and carries a high mortality rate when severe.

Why does an undescended testicle raise the risk so much?

A retained testicle sits at core body temperature instead of the cooler scrotal environment, and that chronic thermal stress is associated with neoplastic change. Classic epidemiologic work found cryptorchid testicles carry up to 13 times the risk of developing a Sertoli cell tumor, which is why removal rather than monitoring is the standard recommendation.

Can a Sertoli cell tumor cause a yeast infection on my dog's skin?

Not directly, but the estrogen-altered skin becomes greasy, thickened, and warm in folds, which favors Malassezia yeast overgrowth in the prepuce, groin, and armpits. That secondary overgrowth persists after castration until it is treated topically, so many dogs need antifungal and antiseptic skin care during the recovery period.

Sources

1. Hayes HM, Pendergrass TW. Canine testicular tumors: epidemiologic features of 410 dogs. International Journal of Cancer. 1976;18(4):482-487.
2. Grieco V, Riccardi E, Greppi GF, et al. Canine testicular tumours: a study on 232 dogs. Journal of Comparative Pathology. 2008;138(2-3):86-89.
3. Sanpera N, Masot N, Janer M, et al. Oestrogen-induced bone marrow aplasia in a dog with a Sertoli cell tumour. Journal of Small Animal Practice. 2002;43(8):365-369.
4. Miller WH, Griffin CE, Campbell KL. Endocrine and metabolic diseases. In: Muller and Kirk's Small Animal Dermatology. 7th ed. Elsevier; 2013:501-553.
5. Liao AT, Chu PY, Yeh LS, et al. A 12-year retrospective study of canine testicular tumors. Journal of Veterinary Medical Science. 2009;71(7):919-923.

Related Reading

• Sex Hormone Dermatosis in Dogs: How Estrogen and Testosterone Affect Skin
• Castration Responsive Dermatosis in Dogs: When Neutering Fixes Skin Problems
• Cushing's vs Hypothyroidism: Comparing Skin Changes in Dogs

Emiel Maddens

Founder of Vetified. Develops topical antifungal and antimicrobial formulations for companion animals. Vetified products are listed on DailyMed and manufactured through FDA-registered facilities in the United States.

Veterinary review: All Vetified content is developed in consultation with licensed veterinary professionals and references peer-reviewed research published in journals including Veterinary Dermatology, JAVMA, and Journal of Small Animal Practice.