By the Pethocyn editorial team. Published October 29, 2026. Updated November 1, 2026. Sources are listed at the end of the article.

Hair loss without itching can come from hormone disease, so your vet rules that out first. An underactive thyroid (hypothyroidism), an overactive adrenal gland (hyperadrenocorticism, Cushing’s disease) and sex hormone imbalance in dogs that are not neutered can all thin the coat in a pattern much like the conditions in this article [1][3]. Alopecia X and seasonal flank alopecia are two causes of patterned hair loss that your vet considers once hormone disease and other causes have been ruled out [2][3][9].

So if your dog’s coat is thinning, see your vet before anything else. For the wider picture of hair loss, see our post on hair loss in dogs.

What these conditions are

Seasonal (recurrent) flank alopecia. Dogs lose hair in patches on the flanks, often in the darker months, and the problem tends to come back in some breeds more than others [2][4]. A university dermatology handbook notes that it is not always seasonal or recurrent, so it prefers the plain name “flank alopecia”, and that its exact cause is still unknown [2].

Alopecia X. The “X” stands for an unknown cause. Over the years it has had many other names, and specialists now often call it hair cycle arrest, after what is seen under the microscope: the hair follicles stop moving into their growing phase [3][6][7]. It is a hair loss problem without skin inflammation, seen most often in Pomeranians [3][7].

Which dogs are affected

  • Flank alopecia: Boxers, French and English Bulldogs and Airedale Terriers have a higher risk, but it has been described in many other breeds, such as Schnauzers, Miniature Poodles and Dobermans [1][2]. Case reports also describe it in Affenpinschers and a Tibetan Terrier [12][13]. Most dogs are first affected between about 3 and 6 years of age [2].
  • Alopecia X: it is seen mainly in Nordic-type breeds such as the Chow Chow, Pomeranian, Keeshond, Samoyed, Alaskan Malamute and Siberian Husky, and also in Miniature Poodles; Pomeranians are over-represented [3]. A 2026 study lists Malamutes, Chow Chows, Schipperkes and Poodles among other breeds it often affects [8], and another found it in many toy poodles as well as Pomeranians [6]. It usually starts in young adults of about 1 to 3 years, in males or females, neutered or not [3].
  • In one study of Pomeranians, dogs with a woolly coat and male dogs were more likely to develop alopecia X [7].

Is it inherited?

Genes likely play a part in both conditions, since some breeds are affected far more often than others and alopecia X can run in related dogs [2][3]. For alopecia X, one study suspects a genetic cause [7], and a report on four related Pomeranians suggested a single dominant gene might be involved while noting that a recessive pattern could not yet be ruled out [14]. For flank alopecia, the 2026 review suggests a mix of causes, with environment, hormones and genes all likely to play a part [4]. Day length seems to matter too [2].

What you may notice

Seasonal flank alopecia

  • Sharply edged bald patches on both sides of the body behind the ribs, often round or with scalloped edges, usually without redness [2][4].
  • The bare skin is often much darker than normal, while the skin and coat around the patches look normal [2][4].
  • Sometimes only one side is affected, or one side more than the other [2].
  • In the northern hemisphere, hair loss usually starts in autumn or winter, when days get shorter, and the hair often grows back by itself in spring or summer [2][4]. The new hair may be a darker colour [2].
  • Not every case follows that pattern: in one report of five spayed females, the hair loss began in spring (March or April) and cleared up by itself in July or August [10].

Alopecia X

  • An early, easily missed sign is a dry, dull coat that loses its longer outer (guard) hairs, so the coat can look puppy-like [1][3].
  • Bald areas then appear on the neck, around the tail and rear end, on the back and on the backs of the thighs, and in time much of the body can lose hair, while the head and lower legs are usually spared [3]. In a report of four Pomeranians, bald patches were seen at the base of the tail, on the back, on the hind legs and on the belly [14].
  • The bare skin often turns dark [3][14], and the hair loss is usually on both sides of the body [6].
  • Hair may regrow at spots where the skin was injured, such as biopsy sites [3].
  • Apart from the hair loss, affected dogs are healthy, and the condition itself does not itch [3]. Skin infections can still develop on top: in a study of toy poodles and Pomeranians, itching and surface skin infections were common [6]. See our post on pyoderma in dogs.

How it is diagnosed

  • Ruling out hormone disease: your vet looks for underactive thyroid, Cushing’s disease and sex hormone problems, which can cause a similar pattern of hair loss [1][2][3]. Blood and urine tests help here. In one study, dogs with recurrent flank alopecia had normal thyroid results, and thyroid blood tests could usually tell them apart from dogs with an underactive thyroid, although a dog can now and then have both conditions [9].
  • Ruling out other skin disease: for alopecia X, the handbook advises several skin scrapings to rule out Demodex mites, and it lists follicular problems (follicular dysplasia) and flank alopecia among the conditions to tell apart [3].
  • History and examination: the diagnosis rests on the dog’s breed and age, the typical pattern and timing of hair loss, and tests that exclude other causes [2][3][4].
  • Skin biopsy: a review of non-itchy hair loss in dogs stresses that the history, a full examination with blood work, well chosen biopsy sites and the biopsy findings all need to be put together to reach a diagnosis [5]. Biopsy findings support these diagnoses but are not specific to them, because hormone diseases can look similar under the microscope [2][3].
  • For alopecia X, there is still no test that can predict which dogs will develop it or prove its cause [7].

Your vet chooses the tests for your dog.

Treatment and care

Your vet first treats any hormone disease or skin infection that is found [1][6].

Both conditions mainly affect the look of the coat, so leaving them untreated is a reasonable choice that your vet may discuss with you [2][3]. For flank alopecia, the handbook says there is no reliably effective treatment, and the 2026 review notes that options range from simply waiting to medical treatment and newer approaches, with little evidence from placebo-controlled trials [2][4]. For alopecia X, the handbook notes that neutering may be suggested for dogs that are not neutered, and that hair loss can come back months or years after any improvement [3]. The toy poodle study suggests that care should aim both at regrowing hair and at restoring the skin’s protective barrier [6].

Your vet will plan care for your dog. Do not give hormones, supplements or other medicines for hair loss unless your vet prescribes them. Some medicines used for alopecia X can have serious side effects [1][3].

Outlook

Dogs with flank alopecia are otherwise healthy, and the 2026 review regards it mainly as a cosmetic problem that does not affect general health [2][4]. Hair often grows back partly or fully the next spring or summer, and the hair loss tends to come and go over the years [4]. Some dogs have only one episode, some skip a year, and in some the hair loss becomes permanent [2]. Outcomes vary between studies:

  • In a report of five spayed females, hair loss first appeared at ages between one and a half and four and a half years [10].
  • In 24 Boxers and Airedale Terriers, the average age at onset was a little over three and a half years [11].
  • In 50 dogs with alopecia X (19 toy poodles, 31 Pomeranians), more than 70% regrew some or all of their hair, across a range of different treatments [6].

These are figures and statements about groups of dogs in particular studies. They do not prove that a treatment caused an outcome, and they cannot predict how one dog will do. Your vet can explain what this means for your dog.

When to see your vet

See your vet if your dog has:

  • thinning hair or bald patches, at any age [1][3];
  • hair loss on both sides of the body, or on the flanks [2][6];
  • darker skin in bald areas [2][14];
  • itching, redness or signs of skin infection [2][6];
  • hair loss together with any other change in your dog’s health, since hormone disease can cause hair loss [1][3].

FAQ

Will the hair grow back?
It varies, and no one can promise it. Flank alopecia often comes and goes with the seasons, but it can become permanent [2][4]; in alopecia X, outcomes varied between dogs in a study, and hair loss can return after regrowth [3][6]. Your vet can tell you what to expect for your dog.

Is it serious?
Both conditions mainly affect the coat, and affected dogs are otherwise healthy [2][3][4]. The important step is ruling out hormone disease and treating any skin infection [1][6].

Does spaying or neutering matter?
In one series of flank alopecia, most affected dogs were spayed females, but the disease was also found in intact females and in intact and neutered males [11]. Alopecia X affects dogs whether or not they are neutered [3]. Your vet can tell you whether this applies to your dog.

Does it happen in cats?
This article covers dogs. For cats, see our post on hair loss in cats.

The bottom line

Alopecia X and seasonal flank alopecia cause patterned hair loss in dogs, mostly in certain breeds [2][3][4]. Because hormone disease can look similar, your vet rules it out first and treats any skin infection [1][9]. Have your vet check a thinning coat.

This article is for general information and does not replace a visit to your veterinarian. If your dog is losing hair, contact your vet.

Sources

  1. Torres SMF. 1.8 Non-Endocrine Alopecias: Learning Objectives and General Considerations. Small and Large Animal Dermatology Handbook, Vol. 2. University of Minnesota, 2025. CC BY 4.0. https://open.lib.umn.edu/animaldermatology2/chapter/non-endocrine-alopecias-general-consideration/
  2. Torres SMF. 1.9 Canine Flank Alopecia. Small and Large Animal Dermatology Handbook, Vol. 2. University of Minnesota, 2025. CC BY 4.0. https://open.lib.umn.edu/animaldermatology2/chapter/canine-flank-alopecia/
  3. Torres SMF. 1.10 Alopecia X. Small and Large Animal Dermatology Handbook, Vol. 2. University of Minnesota, 2025. CC BY 4.0. https://open.lib.umn.edu/animaldermatology2/chapter/alopecia-x/
  4. Verschuuren-Tjoeng MUMY, Schlotter YM, van Geijlswijk IM, et al. Canine flank alopecia: A narrative review of clinical presentation, genetic basis, and therapeutic management. Vet J. 2026;320:106877. https://pubmed.ncbi.nlm.nih.gov/42722190/
  5. Welle MM. Canine noninflammatory alopecia: An approach to its classification and a diagnostic aid. Vet Pathol. 2023;60(6):748-769. https://pubmed.ncbi.nlm.nih.gov/37191329/
  6. Ueyama A, Amatuji H, Iyori K, Hsiao YH. Clinical Characteristics of Hair Cycle Arrest in Toy Poodles: A Retrospective Comparison With Pomeranians. Vet Dermatol. 2026 (online ahead of print). https://pubmed.ncbi.nlm.nih.gov/42885475/
  7. van Hensbergen I, van den Broek J, van Amersfort K, van der Lee A. Evaluation of phenotypic risk indicators for the development of alopecia X (hair cycle arrest) in Pomeranian dogs in the Netherlands and Belgium. Vet Dermatol. 2025;36(2):196-204. https://pubmed.ncbi.nlm.nih.gov/39873118/
  8. Halajian Z, Khoshnegah J, Parham A. Serum insulin-like growth factor-1 concentrations are not reduced in Pomeranian dogs with Alopecia X: a case-control study. BMC Vet Res. 2026;22(1):588. https://pubmed.ncbi.nlm.nih.gov/42443886/
  9. Daminet S, Paradis M. Evaluation of thyroid function in dogs suffering from recurrent flank alopecia. Can Vet J. 2000;41(9):699-703. https://pubmed.ncbi.nlm.nih.gov/10992988/
  10. Scott DW. Seasonal flank alopecia in ovariohysterectomized dogs. Cornell Vet. 1990;80(2):187-195. https://pubmed.ncbi.nlm.nih.gov/2318043/
  11. Miller MA, Dunstan RW. Seasonal flank alopecia in boxers and Airedale terriers: 24 cases (1985-1992). J Am Vet Med Assoc. 1993;203(11):1567-1572. https://pubmed.ncbi.nlm.nih.gov/8288480/
  12. Waldman L. Seasonal flank alopecia in affenpinschers. J Small Anim Pract. 1995;36(6):271-273. https://pubmed.ncbi.nlm.nih.gov/7650925/
  13. Bassett RJ, Burton GG, Robson DC. Recurrent flank alopecia in a Tibetan Terrier. Aust Vet J. 2005;83(5):276-279. https://pubmed.ncbi.nlm.nih.gov/15957387/
  14. Mausberg EM, Drögemüller C, Rüfenacht S, et al. [Inherited alopecia X in Pomeranians]. Dtsch Tierarztl Wochenschr. 2007;114(4):129-134. (Article in German; English abstract.) https://pubmed.ncbi.nlm.nih.gov/17484499/