Hypothyroidism (Canine)

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Hypothyroidism, a common endocrine disease of dogs, is a disorder of the thyroid gland, located in the neck region. Dogs with hypothyroidism have a decreased production and release of thyroid hormones. These hormones play a key role in controlling metabolic rate, affecting many organ systems.

Common names: Underactive thyroid
Scientific names: Hypothyroidism

Diagnosis

Signalment
Hypothyroidism is commonly diagnosed in middle-aged (4-10 years old) dogs. The disease is common in large-breed dogs, affecting males and females equally. Hypothyroidism may be more common in spayed females, compared with intact females. Any breed of dog can develop hypothyroidism, althoughbreeds are predisposed, including the golden retriever, Doberman pinscher, American cocker spaniel, Old English sheepdog, boxer, beagle and Irish setter.

Incidence/prevalence
Hypothyroidism is the most common endocrine (hormonal) disorder seen in dogs.

Geographic distribution
There is no geographic predilection for hypothyroidism.

Clinical signs (primary, most to least frequent, scientific term, synonyms)
Lethargy (decreased energy, less interest in playing, sleeping more), Obesity (sometimes without an increased appetite), Exercise intolerance (becomes tired easily).

Clinical signs (secondary, most to least frequent, scientific term, synonyms)
Alopecia (hair loss, usually on tail and sides of the body), Pyoderma (bacterial infections of the skin), Heat-seeking behavior (looking for warm places to lie down).

Causes (scientific, common term)
Lymphocytic thyroiditis (autoimmune disease), Idiopathic atrophy (degeneration of the gland from an unknown cause).

Organ systems affected (most to least affected)
Endocrine (metabolism), Skin and hair coat, Neuromuscular.

Diagnostic tests
Routine blood work (CBC, biochemical profile and urinalysis), Thyroxine (T4) level and free T4 level, TSH (thyroid stimulating hormone) level.

Differential Diagnosis
Dermatologic (skin) disease, Hypoadrenocorticism (Cushing's disease).

Overview

Hypothyroidism, a common endocrine disease of dogs, is a disorder of the thyroid gland, located in the neck region. Hypothyroidism is the most common endocrine (hormonal) disease diagnosed in canines. Dogs with this condition have impaired production and decreased secretion of thyroid hormones, one of which is thyroxine (T4). These hormones have many functions, including controlling cellular metabolic rate. Affected dogs are usually middle-aged, medium- to large-breed dogs. Males and females are affected equally.

Since dogs with hypothyroidism have a decreased metabolic rate, clinical signs often include decreased energy, weight gain, dry hair coat, and symmetrical hair loss, especially on the tail and sides of the body. These areas of hair loss are not typically itchy unless secondary skin infection has developed. The most common cause of hypothyroidism is autoimmune, in which the body's own immune system attacks the thyroid gland. Idiopathic degeneration, which is deterioration of the thyroid gland from unknown causes, is the second-most-common cause.

Thyroid disease is usually diagnosed from a combination of blood work, clinical signs and response to therapy. Treatment consists of thyroid-hormone-replacement medication.

Treatment

Home care
All prescribed medications must be given as instructed, and the pet should be monitored closely. The medication, especially chewable products, should be kept in a safe place away from the reach of the pet. Although large overdose is well tolerated, excitement and irritability could result. There are no effective ways to prevent this hypothyroidism.

Professional care
A combination of blood work and clinical signs are used to diagnose hypothyroidism. The veterinarian will typically run routine blood work as well as thyroid-specific blood work, including total T4 (thyroxine), free T4 level and TSH (thyroid stimulating hormone).

Once the diagnosis is confirmed, the veterinarian will develop a treatment plan specific for each patient, which likely will include thyroid (thyroxine) supplementation. Thyroid-supplement medications are supplied as pills prescribed daily for the life of the pet.

Action
The diagnosis, treatment plan and follow-up instructions should be developed by a veterinarian. Routine physical exam and follow-up blood work will be periodically obtained to evaluate effectiveness of treatment and to allow for dose adjustments.

Outcome
Uncomplicated hypothyroidism can be treated. Most, if not all, the clinical signs begin to resolve within one or two months after initiation of thyroid supplementation. Adult dogs that are treated appropriately with medications should have a normal life expectancy.

References/Additional Readings
Nelson, Richard W. Endocrine Disorders. In: Nelson, R.W & Couto C.G. Small Animal Internal Medicine 3rd Edition. St. Louis: Mosby, 2003; 691-709.

In: Kahn, C.M The Merck Veterinary Manual 9th Edition. Whitehouse Station: Merck & Co., Inc. 2005; 460 - 466.

Author
Marisa Altieri, DVM

Editor
Steven Hansen, DVM, MS, MBA, DABVT, DABT

Please note that PetVet should only be used as a guide and in no way should be substituted for licensed veterinary care.