Cushing's vs Hypothyroidism: Comparing Skin Changes in Dogs

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

What Is the Difference Between Cushing's Disease and Hypothyroidism in Dogs?

Cushing's disease is an excess of cortisol, and hypothyroidism is a deficiency of thyroid hormone, and although they sit at opposite ends of the endocrine spectrum they produce a strikingly similar skin picture: symmetrical hair loss that does not itch. Cushing's disease, properly called hyperadrenocorticism, occurs when the adrenal glands produce too much cortisol, either because a pituitary tumour is overproducing ACTH (the hormone that stimulates the adrenal glands) or because an adrenal tumour is secreting cortisol autonomously. Hypothyroidism is the opposite problem in a different gland, a failure of the thyroid to produce enough thyroxine to maintain normal metabolic rate.

The overlap in skin signs is not coincidental. Both cortisol excess and thyroid deficiency arrest the hair follicle in telogen, the resting phase of the growth cycle. Hair that is already resting simply falls out over time and is not replaced, which produces gradual, painless thinning rather than the patchy, inflamed, self-traumatised lesions of allergy or parasites. In both diseases the friction points go first: the flanks, the trunk, the tail, and the pressure points, while the head and the lower legs typically keep their coat.

Frequency differs. Hyperadrenocorticism is diagnosed in an estimated 0.1% to 0.2% of the general dog population, with a strong bias toward middle-aged and older dogs, and around 80% to 85% of those cases are pituitary-dependent (Behrend et al., 2013). Hypothyroidism has a reported prevalence in the range of 0.2% to 0.8%, and more than 95% of cases are primary, resulting from lymphocytic thyroiditis or idiopathic follicular atrophy that destroys the gland over months to years (Scott-Moncrieff, 2007).

Distinguishing them matters because the treatments are unrelated and neither is harmless if given to the wrong dog. Thyroid supplementation will not touch a cortisol problem, and adrenal-suppressing drugs given to a hypothyroid dog are dangerous.

What Causes the Skin Changes in Each Disease?

The mechanisms behind the skin changes are entirely different, and the differences are visible to a careful examiner. Cortisol is catabolic, meaning it breaks tissue down. Chronic cortisol excess suppresses fibroblast activity and collagen synthesis, thinning the dermis until the skin becomes soft, slack, and almost translucent. Owners often notice that superficial veins along the abdomen have become unusually easy to see, or that a routine blood draw leaves a bruise that lingers for a week.

Cortisol also plugs the hair follicle opening with keratin and sebum, producing comedones (blackheads), most obviously along the ventral abdomen and around the nipples. In a subset of dogs, calcium salts precipitate into the dermis and form hard, gritty, chalky plaques known as calcinosis cutis, most often along the dorsal midline, the neck, the axillae, and the groin. Calcinosis cutis has been reported in a minority of Cushingoid dogs, and its presence is highly suggestive of hyperadrenocorticism because very few other conditions produce it. It is painful, frequently becomes secondarily infected, and can take months to resolve after cortisol levels are controlled.

Thyroid hormone deficiency does the reverse: it causes accumulation rather than breakdown. Low thyroxine slows epidermal turnover, so keratin and sebum build up on the surface and produce dry, dull, scaly seborrhea. It also allows glycosaminoglycans to deposit in the dermis, drawing water into the tissue and producing a firm, non-pitting thickening called myxedema. In the face, this creates puffiness above the eyes and a drooping of the facial skin that clinicians describe as a tragic facial expression. Hypothyroid dogs also lose the fine guard hairs of the tail first, leaving the smooth, hairless rat tail that is one of the more recognisable signs.

Both hormonal states suppress local skin immunity, which is why recurrent superficial pyoderma and Malassezia overgrowth are so common in endocrine patients, and why a dog whose skin infection keeps returning after a good course of therapy deserves an endocrine workup rather than another round of antibiotics.

Signs and Symptoms: How the Two Conditions Compare Side by Side

The clearest way to separate these diseases at home, before any bloodwork, is to look at the skin texture and the systemic signs rather than the pattern of hair loss, because the hair loss pattern is nearly identical.

How Do Veterinarians Diagnose Cushing's and Hypothyroidism?

Neither disease can be confirmed on physical examination alone, and neither should be diagnosed on a single hormone value taken in isolation. Both diagnoses require compatible clinical signs plus supportive testing, in that order.

For hyperadrenocorticism, routine bloodwork usually comes first and often shows a markedly elevated alkaline phosphatase, a stress leukogram, mild elevations in cholesterol, and dilute urine. Confirmation then relies on dynamic testing. The low dose dexamethasone suppression test (LDDST) has a reported sensitivity of roughly 85% to 100% for pituitary-dependent disease, making it the preferred screening test, but its specificity is considerably lower, in the region of 44% to 73%, so a sick dog with any non-adrenal illness can produce a false positive. The ACTH stimulation test is less sensitive, at roughly 80% to 83% for pituitary-dependent disease and closer to 60% for adrenal tumours, but it is more specific and it is the test used to monitor treatment. Abdominal ultrasound then distinguishes two symmetrically enlarged adrenal glands (pituitary-dependent) from one enlarged mass with an atrophied opposite gland (adrenal-dependent).

For hypothyroidism, total T4 is a reasonable screening test because a normal value largely rules the disease out, but a low value on its own proves very little. Total T4 is suppressed by non-thyroidal illness, by phenobarbital, by sulfonamide antibiotics, and by glucocorticoids, which is exactly the scenario in a chronically itchy dog on steroids. Free T4 by equilibrium dialysis combined with endogenous TSH is the more reliable pairing: the classic confirmatory result is a low free T4 alongside a high TSH. Thyroglobulin autoantibodies support a diagnosis of lymphocytic thyroiditis but do not by themselves indicate current hormone deficiency.

A complicating point worth knowing: cortisol excess suppresses thyroid hormone, so a genuinely Cushingoid dog will frequently return a low T4 and can be misdiagnosed as hypothyroid. When both conditions appear to be present, the adrenal disease is investigated and treated first, and thyroid testing is repeated after cortisol is controlled. Related endocrine hair loss patterns are covered in our guides to growth hormone responsive dermatosis and sex hormone dermatosis.

Treatment Options and Skin Care for Endocrine Dogs

Treatment of the hormonal disorder is what restores the coat, and topical care is what protects the skin while that happens. Neither one substitutes for the other.

Hyperadrenocorticism is most commonly managed with trilostane, a competitive inhibitor of the enzyme 3-beta-hydroxysteroid dehydrogenase, which reduces cortisol synthesis. Typical published starting doses fall in the range of 1 to 3 mg/kg per day, frequently split into twice daily dosing, with ACTH stimulation testing at intervals to confirm the dose is neither insufficient nor excessive. Systemic signs such as thirst and appetite usually improve within two to four weeks, but coat regrowth lags well behind, commonly taking three to six months, and some dogs regrow a coat of noticeably different colour or texture. Adrenal tumours may instead be candidates for surgical adrenalectomy.

Hypothyroidism is treated with synthetic levothyroxine sodium, usually started around 0.02 mg/kg by mouth every 12 hours and adjusted on post-pill T4 monitoring after four to eight weeks. Energy and mental brightness often improve within one to two weeks, dry seborrhea and scaling settle over roughly four to six weeks, and full coat regrowth generally takes three to six months. Treatment is lifelong.

While the endocrine therapy is taking effect, the skin remains immunologically compromised and prone to bacterial overgrowth. This is where topical antiseptic care earns its place. Chlorhexidine Shampoo contains chlorhexidine gluconate 2%, a cationic biguanide that binds to the negatively charged bacterial cell membrane, disrupts membrane integrity, and causes leakage of intracellular contents. Because it also binds to keratin in the stratum corneum, chlorhexidine leaves a residual antibacterial effect on the skin surface after rinsing, which is the property that makes it useful for the repeat surface colonisation seen in endocrine patients rather than a one-off infection. It is an FDA-registered over-the-counter veterinary drug listed on DailyMed, not a cosmetic shampoo.

Practical technique matters more than product choice: work the lather down to the skin rather than over the coat, and leave it in contact for ten minutes before rinsing, because chlorhexidine needs contact time to work. For Cushingoid dogs with thin, fragile skin, avoid vigorous scrubbing and skip any hard-bristled brush. For further reading, see our guides on why skin infections keep coming back and pyoderma in dogs, along with our condition page on bacterial skin infections in dogs.

Prevention and Long-Term Management

Neither condition can be prevented, since both are driven by tumour growth or immune-mediated glandular destruction rather than by anything in the dog's environment or diet. What can be changed is how early they are caught and how well the secondary skin damage is contained.

Early detection is the single most useful intervention. Photograph your dog's flanks, tail, and abdomen every three months once they pass eight years of age. Symmetrical thinning is remarkably easy to miss day to day and remarkably obvious in a six month comparison. Track water intake as well: a healthy dog drinks roughly 50 to 60 ml per kilogram per day, and consistently exceeding 100 ml per kilogram per day warrants investigation.

For dogs already diagnosed, long-term management rests on three habits. First, never adjust the dose independently, because both trilostane underdosing and overdosing carry real consequences, and levothyroxine over-supplementation produces panting, weight loss, and restlessness that mimic Cushing's. Second, keep monitoring bloodwork on schedule; trilostane patients need periodic ACTH stimulation testing, and levothyroxine patients need post-pill T4 checks with the timing of the last dose recorded. Third, treat every secondary skin infection to full resolution rather than to visual improvement, since partially treated pyoderma is the most common reason an endocrine dog's skin never fully settles.

Finally, be patient with the coat. Hair follicles that have been arrested in telogen for a year do not restart synchronously, and regrowth typically appears first at the friction points and last over the flanks. A dog whose energy, thirst, appetite, and skin texture have all normalised is a dog whose treatment is working, even if the coat is still six weeks behind.

Frequently Asked Questions

Can a dog have both Cushing's disease and hypothyroidism at the same time?

Yes, though true concurrent disease is uncommon. Far more often, cortisol excess is artificially suppressing thyroid hormone and producing a falsely low T4. This is called euthyroid sick syndrome. The standard approach is to investigate and control the adrenal disease first, then retest thyroid function once cortisol has been normalised for several weeks.

Does hair grow back after treating Cushing's disease or hypothyroidism in dogs?

In most dogs, yes. Systemic signs such as thirst, appetite, and energy usually improve within two to four weeks, but hair follicles arrested in the resting phase restart slowly. Expect visible regrowth at three months and a substantially restored coat by six months. Some dogs regrow a coat of different colour or texture, which is cosmetic and not a treatment failure.

Why does my hypothyroid dog keep getting skin infections?

Thyroid hormone deficiency slows epidermal turnover and impairs local immune defence, so bacteria and Malassezia yeast colonise the skin surface more readily. Recurrent superficial pyoderma is reported in a substantial proportion of hypothyroid dogs. The infections typically stop recurring once levothyroxine has been at an effective dose for two to three months, with antiseptic bathing bridging the gap.

What is calcinosis cutis and is it dangerous?

Calcinosis cutis is the deposition of calcium salts in the dermis, producing hard, gritty, chalky plaques most often along the back, the neck, the armpits, and the groin. It is strongly associated with hyperadrenocorticism and with prolonged steroid administration. It is painful, frequently becomes secondarily infected, and can take several months to resolve after cortisol levels are controlled. It always warrants veterinary assessment.

How often should I bathe a dog with an endocrine skin disorder?

Twice weekly antiseptic bathing is a common starting frequency while a secondary infection is active, reducing to once weekly or once every two weeks for maintenance. Leave the lather in contact with the skin for ten minutes before rinsing, since chlorhexidine requires contact time. For Cushingoid dogs with fragile skin, use gentle hand lathering and avoid stiff brushes or aggressive towel drying.

Sources

1. Behrend EN, Kooistra HS, Nelson R, Reusch CE, Scott-Moncrieff JC. Diagnosis of spontaneous canine hyperadrenocorticism: 2012 ACVIM Consensus Statement (small animal). Journal of Veterinary Internal Medicine. 2013;27(6):1292-1304.
2. Scott-Moncrieff JC. Clinical signs and concurrent diseases of hypothyroidism in dogs and cats. Veterinary Clinics of North America: Small Animal Practice. 2007;37(4):709-722.
3. Dixon RM, Reid SWJ, Mooney CT. Epidemiological, clinical, haematological and biochemical characteristics of canine hypothyroidism. Veterinary Record. 1999;145(17):481-487.
4. Zur G, White SD. Hyperadrenocorticism in 10 dogs with skin lesions as the only presenting clinical signs. Journal of the American Animal Hospital Association. 2011;47(6):419-427.
5. Miller WH, Griffin CE, Campbell KL. Endocrine and metabolic diseases. In: Muller and Kirk's Small Animal Dermatology. 7th ed. St Louis: Elsevier; 2013:501-553.

Related Reading

• Growth Hormone Responsive Dermatosis in Dogs: Hair Loss Without Itch
• Sex Hormone Dermatosis in Dogs: How Estrogen and Testosterone Affect Skin
• Why Does My Dog Keep Getting Skin Infections? Causes and Solutions
• Acanthosis Nigricans in Dogs: Causes & Treatment

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.