1.1 Endocrine Alopecias – Learning Objectives
Learning Objectives
- Know! Hormones affect many tissues and organ systems; therefore, diseases causing endocrine alopecias are considered syndromes. Some of the non-cutaneous clinical signs are specific and can help achieve a diagnosis.
- Remember! The alopecia associated with endocrine diseases is non-inflammatory and, as such, non-pruritic. Horses with pituitary pars intermedia dysfunction (PPID) or hyperadrenocorticism develop hirsutism (long hair) and not alopecia.
- Remember! Dogs with endocrine alopecias are more prone to develop secondary bacterial and/or yeast overgrowth/infection than cats and large animals. The secondary overgrowth/infection may lead to the development of pruritus. It is important to identify these secondary problems and manage them properly.
- Important! False positive results can occur with the tests available to diagnose the endocrine diseases discussed in this section; therefore, only test the patient if the history and clinical signs are very suggestive of the disease.
- Canine Hyperadrenocorticism (HAC) – Know! About 80% of dogs with HAC have a functional pituitary tumor and about 20% have a functional adrenal tumor. The pituitary tumor is typically a microadenoma and the adrenal tumor can be an adenoma or adenocarcinoma.
- Canine HAC – Know! Currently, the low-dose dexamethasone suppression test (LDDST) and the ACTH stimulation test are used to confirm a presumptive clinical diagnosis of HAC in the dog. The LDDST is more sensitive but the ACTH stimulation test is more specific. Make sure to understand the pros and cons of these tests and how to interpret them.
- Canine HAC – Keep in mid! The urine cortisol:creatinine ratio test is sensitive but non-specific. Nevertheless, it has a very good negative predictive value allowing you to request this test if you do not think HAC is likely but want to rule it out.
- Canine HAC – Remember! After you diagnosed canine HAC, you have to localize the disease either to the pituitary gland (~80%) or the adrenal gland (~20%). This is important to inform the pet owner about prognosis and treatment options. Abdominal ultrasound and endogenous ACTH concentration are currently considered the best tests.
- Canine HAC – Be aware! Currently, the most effective treatments are trilostane and mitotane. The mechanism of action of these drugs is different but ultimately both reduce the serum concentration of cortisol and consequently ameliorate clinical signs of the disease. However, these treatments do not directly target the causes of HAC. Many clinicians prefer trilostane over mitotane because of its reversible effect. Both treatments can result in significant decrease in serum cortisol and mineralocorticoid levels and close monitoring throughout treatment is mandatory. Make sure to review treatment details and monitoring protocols in the canine HAC section.
- Feline HAC – Know! HAC occurs much less frequently in cats than in dogs. Similar to dogs, about 80% of cats have a functional pituitary tumor and about 20% have an adrenal tumor. Most cats with HAC (80-90%) will have concurrent diabetes mellitus, in contrast to dogs (~5-10%).
- Feline HAC – Keep in mind! Many of the clinical signs in cats and dogs are similar and they result from the catabolic effects of cortisol in protein, carbohydrate, and lipids. However, about 15-50% of cats with HAC will develop skin fragility, which is not seem in dogs.
- Feline HAC – Know! The best test to confirm a presumptive clinical diagnosis of HAC is the LDDST. However, the dose of dexamethasone is much higher in cats (i.e. 0.1mg/kg IV) than the dose used in dogs (i.e. 0.01 mg/kg IV). The ACTH stimulation test is not recommended because it has low sensitivity. Understand the interpretation of the LDDST.
- Feline HAC – Know! Similar to dogs the urine cortisol:creatinine ratio should only be requested if the index of suspicion for HAC is low because this test is associated with many false positive results. However, similarly to dogs, it has a very good negative predictive value.
- Feline HAC – Know! After a diagnosis is confirmed, you need to localize the disease to discuss with the pet owner treatment options and prognosis. The high-dose dexamethasone suppression test has shown to have a sensitivity of 54% in localizing the disease at the pituitary gland. The 4-hour test is as sensitive as the 8-hour test. If no suppression is noted at 4-hour (or 8-hour), the disease cannot be localized. Similar to dogs, abdominal ultrasound and endogenous ACTH concentration are considered the best tests to localize the disease. Be aware of the pros and cons of the localizing/differentiating tests.
- Feline HAC – Know! The fact that most cats with HAC have concurrent diabetes mellitus makes it a more challenging disease to manage. Adrenalectomy has provided the best results in managing HAC in cats if the post-surgical hypoadrenocorticism is properly managed. Currently, few cats have been treated with trilostane but the results appear promising. Be aware of the recommended treatment protocol of trilostane and the complications of surgical treatment.
- Equine Pituitary Pars Intermedia Dysfunction (PPID) – Know! In horses, PPID is associated with a dysfunction of melanotropes in the pars intermedia of the pituitary gland. The disease affects 15-20% of horses 15 years of age or older.
- PPID – Be aware! A clinical sign that should increase your index of suspicion for PPID is regrowth of long hair after a normal shed or failure to shed a longer than normal winter coat. Thus, horses will not develop alopecia, as dogs and cats, but a long hair coat (i.e. hirsutism). Be familiar with the other clinical signs of PPID.
- PPID – Know! To confirm a clinical diagnosis of PPID you have to take into consideration the following (i) the horse’s history and clinical signs, (ii) plasma endogenous ACTH concentration, (ii) overnight dexamethasone suppression test, and/or (iii) the TRH stimulation test. Be familiar with the pros and cons of each of these tests.
- PPID – Know! Pergolide mesylate is currently the mainstay treatment for PPID. Response to therapy is typically seen in 30 days. Chronic cases may not respond as well. Cyproheptadine can be added to the treatment regimen to help control severe cases where the response to pergolide is less than desirable.
- Canine hypothyroidism – Know! The most common cause of hypothyroidism in dogs is lymphocytic thyroiditis and idiopathic atrophy of the thyroid gland.
- Canine hypothyroidism – Remember! Before a dog develops clinical signs of hypothyroidism, more than 75% of the parenchyma of both thyroid lobules needs to be affected and the process is slow. Therefore, the age range of dogs with hypothyroidism is about 4-8 years. Keep in mind that a dog can have lymphocytic hypothyroidism or idiopathic atrophy of the thyroid parenchyma and never develop hypothyroidism.
- Canine hypothyroidism – Know! Thyroid hormones affect the metabolism of almost every cell in the body! Common signs of hypothyroidism are exercise intolerance, heat seeking, and increase in body weight without eating more, all reflecting the patient’s low metabolic rate. However, owners may relate them to old age and not perceive these clinical signs as important. Thus, investigate the presence of these clinical signs if the owner does not present them as concerns during history taking. Non-inflammatory alopecia that spares the head and distal extremities (except the dorsal muzzle in some dogs and caudal thighs) is also a common sign of canine hypothyroidism and it is often the reason pet owners seek veterinary care. Be familiar with other clinical signs associated with canine hypothyroidism.
- Canine hypothyroidism – Keep in mind! Hypothyroidism is over diagnosed because many veterinarians base the diagnosis primarily on the serum concentrations of thyroid hormones. However, these tests are non-specific and can be affected by various drugs and non-thyroidal diseases (typically severe illness)! Therefore, only test dogs for hypothyroidism if the history and clinical signs are suggestive, the dog is not suffering of severe non-thyroidal disease, and not taking any drugs that can spuriously decrease total T4 and free T4 by equilibrium dialyses (fT4d).
- Canine hypothyroidism – Know! About 70-80% of hypothyroid dogs have high cholesterol and triglycerides serum levels. Therefore, elevated fasted cholesterol and triglycerides will support a diagnosis of hypothyroidism in dogs with characteristic history and clinical signs. Endogenous serum TSH is not reliable as a sole test because about one third of dogs with hypothyroidism will have normal endogenous TSH. The best diagnostic tests currently available are total T4 and fT4d. Again, these tests can be affected by severe non-thyroidal and drugs (total T4 > fT4d). TSH is less affected by diseases and drugs and can be combined with total T4 and fT4d to increase their specificity. Be familiar with the other tests available to diagnose hypothyroidism.
- Canine hypothyroidism – Know! Sodium levothyroxine is the recommended treatment for hypothyroidism. Lethargy and exercise intolerance typically resolve within 7-10 days of starting therapy. Non-inflammatory alopecia may take at least 3 months to resolve. Be familiar with the treatment details including “treatment trial” and monitoring protocols.
- Large animals hypothyroidism – Be aware! Hypothyroidism is very rare in large animals. Review this condition in large animal species in the section titled “Hypothyroidism – Large Animals”.
- Canine sex hormone dermatoses are typically associated with hyperestrogenism, hypertestosteronism, and hyperprogesteronism. Consider these conditions as differential diagnosis in intact dogs with non-inflammatory often symmetrical alopecia where hyperadrenocorticism and hypothyroidism have been ruled out. Review these endocrine diseases in the section titled “Sex Hormone Dermatoses”.
