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Endocrine alopecia

in dogs

Fabia Scarampella,

DVM, MSc, Dipl.ECVD

Studio Dermatologico Veterinario,

Milan, Italy

Dr. Scarampella received her diploma

from the European College of

Veterinary Dermatology in 2000 and

in 2007 achieved a Master’s Diploma in Evidence Based

Medicine and Health Care Research Methodology. Co-

author of the book “Manuale pratico di Dermatologia

Veterinaria” she has authored textbooks and scientific

journals. She is currently President of the Italian Veterinary

Dermatology Society (SIDEV), scientific co-ordinator of the

Dermatology Itinerary at the Scuola di Formazione

Veterinaria Post Universitaria, and works as a self-

employed professional dealing exclusively with veterinary

dermatology. She is also a consultant and co-creator of the

veterinary dermatology website www.teledermvet.com.

Introduction

The hair follicle is influenced by hormonal activity

in a variety of ways and is capable in its own right

of synthesizing and converting a wide range of

hormones. In the rat, it has been demonstrated

that estradiol, testosterone and the steroids

produced by the adrenal glands delay the start of

the anagen (active growth) phase, while thyroid

hormones accelerate follicular activity (1). In the

dog, certain endocrinopathies (hypothyroidism,

hypercortisolism), functional neoplasia of the

gonads (hyperestrogenism) and certain follicle

dysplasias (cyclic alopecia of the groin) are

associated with alterations of the follicular cycle

and present as symmetrical alopecia.

The aim of this article is to provide an up-to-date

summary on the pathogenesis and the dermato-

logical signs of hormonal diseases that cause

alterations in the follicular cycle, and offer a

diagnostic approach for dogs with non-pruritic

symmetrical alopecia.

KEY POINTS

Non-pruritic symmetrical alopecia in dogs is a

clinical condition commonly associated with

endocrinopathies, functional neoplasia of the

gonads, follicular dysplasia and idiopathic alopecia.

Although unlikely, infectious conditions that can

affect the hair follicle, in particular demodicosis,

should be excluded.

Ultrasonography is advisable for female dogs

presenting with enlarged vulva and teats and for

male dogs that are either cryptorchid or have

alterations in the size/consistency of the testicles.

The initial laboratory examinations should include

hematology, CBC and urine examination.

Dynamic assessment of thyroid function must be

carried out after any adrenal function test.

Cutaneous biopsy may help identify follicular

dysplasia or an idiopathic alopecia (Alopecia X).

Hypothyroidism

Thyroid hormone receptors are present in all

tissues. In the skin, receptors are especially abund-

ant in the sebocytes, the cells of the external follicle

sheath, and the dermal papilla. Because of this, the

thyroid exercises significant influence on the hair

follicle cycle and production of sebum. Thyroid

hormones also control lipogenesis and serum and

cutaneous levels of fatty acids; additionally they

stimulate the proliferation of fibroblasts and the

synthesis of collagen, and (via the regulation of the

synthesis and catabolism of the glycosamino-

glycans) influence the thickness of the dermis.

The thyroid produces thyroxin (T4) and tri-

iodothyronine (T3) in a ratio of 4:1 and in the

circulation these hormones are mainly bound to

plasma proteins. This assists with storing and

regulating the serum concentration of the hormones.

Less than 0.05% of T4 and less than 0.5% of T3 are

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ENDOCRINE ALOPECIA IN DOGS

present in the blood in free form (2). The free

form of T4 (fT4) penetrates the target cell where

80% converts to the free form of T3 (fT3), a

biologically active form.

increases if combined with data obtained from the

clinical examination. Cutaneous alterations are

reported in 60-80% of dogs with hypothyroidism

(3-4) and include bilaterally symmetric alopecia;

hair that is dull and easily epilated, and which

regrows more slowly after clipping (5); skin

that is hyperpigmented and cold to the touch;

desquamative dermatitis variably associated with

signs of follicular keratosis (follicular casts); and

ceruminous otitis externa. Hypotrichosis and

alopecia are particularly evident in the areas

subject to major trauma such as pressure points,

the perineum, the tail, the bridge of the nose and

the neck (Figure 1) . Note that Irish setters may

show hypertrichosis due to retention of hairs in the

telogen (resting) phase and in these subjects the coat

can turn lighter. The loss of color is probably due to

the effect of environmental factors on hair that is

retained longer than normal within the hair follicles.

Hypothyroidism is the most common endocrinopathy

in dogs. This disease can have a congenital or

acquired origin. Acquired hypothyroidism can be

primary or secondary. Primary hypothyroidism is

due to insufficient production of hormones by the

thyroid glands, while in the secondary form

glandular dysfunction is the result of insufficient

production of pituitary TSH.

An auto-immune thyroiditis is by far the most

common cause of acquired primary hypothyroidism

and is often associated with the presence of

circulating thyroglobulin antibodies (AbTG).

However, the presence of AbTG alone does not

confirm thyroid dysfunction. The second cause of

acquired primary hypothyroidism is idiopathic

thyroid degeneration in which a gradual replace-

ment of the glandular parenchyma with adipose/

fibrous tissue is observed in the absence of an

inflammatory infiltrate. Secondary hypothyroidism

is very rare and has been observed in combination

with pituitary neoplasia. In these cases, hypo-

thyroidism is accompanied by signs related to

concurrent illnesses such as such as Addison’s

disease, diabetes insipidus and reproductive

dysfunction.

The lack of thyroid hormone also leads to reduced

fibroblast activity and alteration of collagen

metabolism. Cutaneous wounds heal more slowly

and in areas subject to trauma an excessive

deposition of fibrous tissue (corns) may be

observed. In hypothyroid dogs, due to the slowed

glycosaminoglycan catabolism, excess hyaluronic

acid accumulates in the dermis, retaining water.

The myxedema that results gives the skin a greater

thickness and a lower temperature.

Among the systematic clinical signs of hypo-

thyroidism the most common are sleepiness, low

resistance to physical exercise and a tendency of

subjects to suffer more from the cold. Bradycardia

The clinical signs observed in hypothyroid

subjects are fundamental for the diagnosis, and

the predictive value of hormone functionality tests

Figure 1.

Hypothyroidism in a female golden retriever (left). Note the perineal alopecia (right).

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Figure 2.

Figure 3.

Symmetrical bilateral alopecia in a 7-year-old Shih Tzu with

hypercortisolism.

Alopecia, comedones and thinning of the ventral skin in the

same dog.

is a frequent clinical sign. Ocular alterations such

as corneal lipidosis, keratoconjunctivitis sicca and

corneal ulceration may also be observed. Hypo-

thyroid bitches often have persistent anestrus and

infertility, while males can have spermatogenic

defects. Gynecomastia and galactorrhea are

observed in 25% of non-sterilized bitches and

occasionally in neutered bitches and males. This

phenomenon is probably due to an increase in the

serum levels of prolactin induced by TRH over-

production. Hypothyroid subjects often have

mild, non-regenerative anemia and defects in

platelet function. Recurrent pyoderma is also a

common complication.

hyperplasia of both adrenal glands, in particular

in the fasciculata and reticularis zone; the adrenal

glands then produce excessive quantities of

cortisol.

Functional adrenal neoplasias represent 15-20%

of spontaneous hypercortisolism cases in dogs.

These neoplasms (adenomas or adenocarci-

nomas) are usually unilateral; unresponsive to the

hypothalamic-pituitary axis control mechanism,

they autonomously produce large quantities of

cortisol whilst the contralateral adrenal gland

usually atrophies.

Iatrogenic hypercortisolism due to prolonged oral,

parenteral or topical administration of cortisone

will show the majority of clinical signs that are

observed with the spontaneous form of the

disease, as well as suppression of the adrenal

functions and elevation of hepatic enzymes.

Hypercortisolism

Glucocorticoids influence hair growth and follicle

pigmentation by means of specific intracellular

receptors present within the cells of the inter-

follicle epidermis and the basal cells of hair

follicles. Although it has been demonstrated in

mice that suppression of these hormones has a

marked effect on follicle growth (6) the exact

molecular mechanism by which this effect is

produced in dogs is not yet known. Hypercortisolism

may be due to excessive production of cortisol by

the adrenal glands (spontaneous hypercortisolism);

or follow administration of excessive quantities

of cortisol (iatrogenic form). In spontaneous

hypercortisolism, the disease is caused by a

neoplasm of the frontal lobe or, more rarely, the

intermediate lobe, of the pituitary or by the

presence of adrenal tumors. The pituitary form

accounts for 80-85% of all cases of spontaneous

hypercortisolism. Here the pituitary produces an

excessive quantity of ACTH and causes bilateral

Spontaneous hypercortisolism is seen in adult

dogs but especially older animals. Clinical signs

are largely the result of excessive quantities

of cortisol. These signs are numerous and their

appearance is conditioned by many factors, in

particular by the location of the neoplasia

(pituitary or adrenal), the age of the subject and

the breed affected. In dogs affected by a pituitary

adenoma the clinical signs tend to manifest

themselves gradually, while in dogs affected by a

pituitary carcinoma or adrenal neoplasm the

appearance of clinical signs are much faster.

Subjects of advanced age are more sensitive to

the catabolic effects of the glucocorticoids and

manifest symptoms more quickly than young

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ENDOCRINE ALOPECIA IN DOGS

Figure 5.

Alopecia and gynecomastia in a bulldog with ovarian cysts.

Figure 4.

Ulcerated firm plaques on the neck of a dog with cutaneous

calcinosis (iatrogenic hypercortisolism).

subjects. Small-sized breeds tend to manifest the

disease in the classic manner and with a greater

number of clinical signs than large-sized breeds.

of the anagen phase and tend to keep the follicle in

the telogen phase (7-8).

Hyperestrogenism as a cause of alopecia is only

reported in the dog and results most commonly

from ovarian cysts; less often the cause is an

estrogen-secreting ovarian or testicular neoplasia.

Hyperestrogenism secondary to ovarian cysts is

most common in the British bulldog and manifests

itself in adults (2-7 years). On the other hand

hyperestrogenism secondary to ovarian neoplasia

has no breed predisposition and manifests itself in

subjects of advanced age.

The presence of PU/PD is the most common initial

symptom and can precede the typical cutaneous

manifestations of the disease by 6-12 months.

Cutaneous signs such as loss of hair sheen,

presence of dry scale, and a slight hypotricosis may

be noted initially but in time alterations in coat

pigmentation and bilaterally symmetric alopecia

can be observed (Figure 2) . The skin of dogs

affected by hypercortisolism is thin, hypotonic and

non-elastic (Figure 3) . Phlebectasia, the presence

of ecchymosis and petechiae after slight trauma,

scaling, comedones and (especially in the iatrogenic

form) cutaneous calcinosis may all be observed.

Cutaneous calcinosis is commonly observed on the

back of the neck, the axillae and the inguinal

regions and manifests itself as hard, whitish

papules and plaques. In time the plaques become

ulcerated and very pruritic (Figure 4) . Pyoderma

and demodicosis are complications commonly

associated with this hormonal disease. Pyoderma

in dogs affected by hypercortisolism typically

manifests itself with the presence of elongated,

non-follicular pustules and mild inflammation.

Infections respond poorly to antibiotic treatment.

The typical clinical signs of both conditions in the

female are bilateral symmetrical alopecia of the

perineum and the inguinal/groin regions. In

chronic cases, alopecia can affect friction points

( e.g. collar) and may eventually affect the whole

trunk. On the skin of the abdomen and the vulva

numerous comedones are often present, and hair

is dry and dull. Bitches often present with estral

cycle abnormalities, gynecomastia and vulva

enlargement (Figure 5) . Endometritis and sub-

sequently pyometritis are common.

Sertolioma, a tumor of the Sertoli cells in the

testes, can induce the appearance of alopecia and

a feminization syndrome in around 1/3 of subjects

affected. These dogs have high levels of circulating

estrogens and the effects of this alteration are

cutaneous, hematological, prostatic and behavioral

abnormalities. There is no breed predisposition

and the neoplastic disease is manifested in adult,

usually older, dogs. The distribution of the

alopecia is similar to that described for females.

Hyperestrogenism

Estrogens are mainly produced by the ovary and,

in both sexes, the adrenal cortex, the liver and the

mammary glands. Hair follicles are themselves

capable of synthesizing estrogens from androgens

by enzymatic action. Estrogens are powerful

modulators of follicle growth and delay the start

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Figure 7.

Figure 6.

Estrogen-secreting testicular neoplasia: alopecia and teat

enlargement in a dog with Sertolioma.

Estrogen-secreting testicular neoplasia. Note the erythematous

linear dermatitis along the prepuce.

The skin in the affected areas is normally hyper-

pigmented and the hair dry, dull and woolly. A

suggestive cutaneous sign is a linear erythema

along the prepuce and the scrotum (Figures 6 and

7) . Other signs are increased teat size, a pendulous

prepuce, little interest in females in heat, prostatic

hypertrophy, thrombocytopenia and anemia.

it is advisable to perform a skin biopsy. Histological

examination will help confirm a suspected follicular

dysplasia associated with pigmentation anomalies

or a recurrent flank alopecia.

However if the problem persists, or is associated

with systemic signs such as polyuria and/or

polyphagia, or if on microscopic examination hair

bulbs are observed to be mostly in the telogen

phase, the presence of hypercortisolism and hypo-

thyroidism must be considered. Initial laboratory

examination should include hematology, CBC and

urinalysis. These tests may demonstrate anomalies

consistent with endocrinopathies and exclude

other diseases such as chronic kidney disease,

diabetes mellitus and certain hepatic diseases.

Note that in subjects where clinicopathological

alterations suggest an endocrinopathy, assessment

of the thyroid function must always be carried

out after adrenal function tests because an

increase in plasma cortisol may reduce serum

T4 levels.

Diagnostic protocol for non-

pruritic symmetrical alopecia

When presented with a case of non-pruritic

symmetrical alopecia, initially all bacterial, fungal

and parasitic diseases that can affect the hair

follicle should be considered. Microscopic exam-

ination of the hair at this stage of the diagnostic

procedure can be useful and is summarized in

Table 1 . If these causes can be excluded, the presence

of metabolic diseases, functional neoplasia of the

gonads (ovarian neoplasia or Sertolioma), ovarian

cysts and follicular dysplasia must be considered.

A general physical examination should be

performed; if alterations are observed in the

dimensions and consistency of the testicles of a

male dog, or if there is a persistent increase in

size of the vulva and teats and alterations in the

estral cycle in a female, it is advisable to request

a testicular or abdominal ultrasound with

possibly biopsy of the gonads.

The main clinico-pathological alterations encountered

in dogs with hypercortisolism and hypothyroidism

are presented in Table 2 . If hypercortisolism is

suspected, endocrine screening tests should be

considered (9-10) (see Cut-out and keep on

pages 47-48 ):

If microscopic examination detects alterations of the

hair shaft, such as deformations of the cortex and

presence of melanin (macro-melanosomes), or if the

alopecia is seasonal and located in the groin region,

Note false positives are possible for each of these

tests and it is extremely important that results are

interpreted in conjunction with the history and

clinical condition of the patient. In cases where a

44 / Veterinary Focus / Vol 21 No 1 / 2011

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