Introduction

Hypothyroidism is a condition that develops as a result of abnormally low levels of circulating thyroid hormones. The hormones produced by the thyroid gland are essential to a number of metabolic processes within the body. Hypothyroidism is a fairly common disorder in domestic dogs, and the symptoms of the condition can mimic those of many other diseases.

Symptoms of Hypothyroidism in Dogs

Hypothyroidism occurs most frequently in large, middle-aged dogs of either gender and of any breed or mixed breed. The symptoms of hypothyroidism are often nonspecific and quite gradual in onset, and they frequently vary based upon the dog’s breed and age at the time of onset of thyroid hormone deficiency. Although the signs of hypothyroidism can be subtle, most affected dogs have one or more of the following symptoms:

• Mental dullness
• Lethargy; listlessness
• Exercise intolerance
• Reluctance to engage in normal activities
• Intolerance to cold (“heat-seeking” behavior)
• Weight gain, without increased appetite or food consumption
• Symmetrical hair loss (alopecia), without itchiness (bilaterally symmetric, nonpruitic truncal alopecia; the head and legs are often spared)
• Excessive shedding
• Greasy skin; flaky skin (seborrhea)
• Dandruff
• Pimples or other pustules on the skin (pyoderma)
• Chronic ear infections (otitis)
• Dry, brittle hair coat
• Skin thickening, especially on the face and forehead (myxedema), giving a puffy appearance referred to as a “tragic facial expression”

The dog’s neuromuscular, reproductive, cardiovascular and/or gastrointestinal systems may be affected as well, causing one or more of the following symptoms:

• Slow heart rate (bradycardia)
• Constipation
• Diarrhea
• Incoordination (ataxia)
• Seizures
• Heart arrhythmias (irregular heart rhythms)
• Anemia
• Infertility
• Decreased libido (reduced sex drive)
• Abortion
• Testicular atrophy

These more generalized symptoms are suggestive of hypothyroidism especially when they accompany the more specific, primarily dermatologic symptoms mentioned above.

Signs of altered metabolism may not be appreciated by owners until thyroid hormone supplementation is started. Weight gain obviously can be attributed to overeating and a lack of sufficient exercise. However, if a dog’s diet and exercise routine have not changed, and the dog is gaining weight for no apparent reason, hypothyroidism should be on the list of suspects. Similarly, hypothyroidism should be considered when a dog develops unexplained skin and coat abnormalities, with hair thinning on the back and around the tail, excessive shedding, greasy flaky skin or other dermatologic disorders.

Dogs at Increased Risk

Hypothyroidism occurs most frequently in large and giant breed, middle-aged dogs (2 to 8 years of age), of either gender and of any breed or mixed breed. Spayed females seem to have a greater risk of developing hypothyroidism than do intact females, although the reason for this association is unclear. Breeds reported to be predisposed to hypothyroidism include the Doberman Pinscher, Golden Retriever, Labrador Retriever, Greyhound, Scottish Deerhound, Great Dane, Irish Setter, Poodle, Cocker Spaniel, Shetland Sheepdog, Airedale Terrier, Schnauzer, Malamute, Boxer, Dachshund, and some other Terriers, Spaniels and Pointers.