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CANINE HYPOTHYROIDISM
Hypothyroidism is the clinical condition caused by thyroid hormone deficiency. The thyroid hormones,
thyroxine (T4) and triiodothyronine (T3), are produced by the thyroid glands located in the neck on
either side of the trachea (windpipe). Thyroid hormones influence the metabolism of most of the
organs in the body. Deficiency of thyroid hormone results in a decreased metabolic rate, causing a
wide variety of clinical signs. “Decreased in metabolic rate” means the speed at which cells function or
“work” slows down. For example, the heart rate slows, mental function slows, body temperature
decreases, and skin/haircoat changes may develop. Thyroid hormone also affects many organ and
cellular function, including immune system function, red blood cell production, and lipid (aka: fat)
metabolism.
Decreased secretion of thyroid hormones by the thyroid gland may be primary, secondary, or tertiary:
A. Primary: results from thyroid gland inflammation (thyroiditis) or progressive failure (atrophy)
of the thyroid glands. In rare cases, thyroid tumors may cause hypothyroidism.
B. Secondary & Tertiary: result from abnormalities in the pituitary gland and hypothalamus,
respectively.
Hypothyroidism is one of the most common endocrine disease of dogs. Breeds predisposed to
developing hypothyroidism include the Golden Retriever, Labrador Retriever, and the Doberman
Pinscher, but any breed of dog may be affected.
The most common clinical signs of hypothyroidism are
 Weight gain
 Cold intolerance
 Lethargy
 Variety of skin problems. The most common skin abnormalities include:
o Symmetrical hair loss on back and around tail base
o Changes in hair color and quality
o Predisposition to skin infections
 Other less common clinical signs include abnormalities of the reproductive and
nervous systems.
Common biochemical abnormalities include:
 Increased cholesterol
 Increased triglycerides
 Liver enzyme elevations
 Anemia
A diagnosis of hypothyroidism is made by performing a blood test called a thyroid profile.
Hypothyroidism can not be diagnosed by simple determination of thyroxine level, as T 4 is potentially
affected by numerous factors. For example, illness and certain medications may lower thyroxine
levels, creating the biochemical appearance of hypothyroidism even though the thyroid glands are
normal. Conversely, T4 levels may appear normal in hypothyroid dogs with elevated anti-thyroid
antibody levels.
Hypothyroidism is an easily treated disease. Treatment involves daily or twice-daily oral medication
with synthetic thyroxine. In most cases, treatment is required for the life of the dog, and is well
tolerated. It may take several weeks for the clinical signs of hypothyroidism to resolve. An increase in
activity level is typically observed after 1 to 2 weeks of treatment. It usually takes longer for skin
problems to resolve, but improvement should be observed within 6 to 8 weeks. In some cases, the
skin may actually appear worse for the first 1 to 2 weeks of treatment as the old haircoat is shed. If a
dog was experiencing reproductive or neurological problems, it may take several months of treatment
for complete resolution of the problems.
Monitoring
In order to establish an appropriate dose of thyroid hormone supplementation, it is recommended
blood samples be initially collected for measurement of thyroid hormone concentrations 1 month after
the initiation of treatment. The results of these tests are used to adjust the dose of thyroid hormone
supplementation. This blood test is always performed 4-6 hours after the morning dose. It is usually
necessary to measure thyroid hormone concentrations every 3 months or if clinical signs of
hypothyroidism recur.
Clinical signs of excessive supplementation with thyroid hormone include:
 Nervousness
 Weight loss
 Increased drinking
 Increased urination
These signs are rare, but if they occur, it is important to contact the doctors and staff of VCA Veterinary
Referral Associates so your pet’s dose may be adjusted.