By Dr. Cindy Krane, DVM

Calusa Veterinary Center

Hypothyroidism is the most common hormonal problem in dogs.  It occurs most often in large breed and giant dogs that are middle aged to senior.  The breeds most commonly afflicted include Golden Retrievers, Doberman Pinschers, Boxers, Dachshunds, Miniature Schnauzers, Poodles, Great Danes and Irish Setters. There is no known sex predilection so males and females are equally affected. 

The thyroid gland is a bi-lobed gland on the neck located either side of trachea. Its primary role is to secrete thyroid hormones.  Two forms are produced; T4 and T3.  T4 is subdivided into T4 (bound) and Free T4.  The free T4 is the form that is converted into T3 in the tissues.  This T3 is the active form of the hormone. The production of T4 is regulated by the pituitary gland.  The pituitary gland (at the base of the brain) secretes TSH (thyroid stimulating hormone) which stimulates the thyroid gland to secrete T4. The thyroid gland in turn controls the metabolic rate of most organs in the body. Inadequate or deficient levels of thyroid hormones cause the disease process referred to as hypothyroidism.

There are multiple forms of hypothyroidism. Primary hypothyroidism accounts for 90% of cases. It includes lymphocytic thyroiditis (an immune mediated destruction of the thyroid gland), idiopathic atrophy (a shrinking of the gland where the cause is unknown) and cancer of the thyroid gland. The other 10% of cases include congenital defects, problems within other glands in the body (the hypothalamus or pituitary), and other rare causes.

The most common clinical signs of hypothyroidism are associated with skin disorders. Typically the dogs coat will be affected.  The coat changes tend to be symmetrical and most prominent along the trunk. The fur may change in color, texture (become dry and brittle) and may shed excessively, thin out or be lost (alopecia).

Often skin changes occur.  The skin can become darker (hyperpigmented), thicker (especially around the folds in the face causing a sad or “tragic” face) and frequently infected. Many dogs with hypothyroidism are obese and fatigue easily, sleep excessively and are intolerant to exercise.  Many have behavioral changes ranging from dull and uninvolved to aggressive. Less frequently they can have heart problems (slow or arrhythmic heart rate), eye problems or neurological problems.

If you suspect your dog may be suffering from hypothyroidism a physical examination and blood sampling is indicated. The diagnosis is made foremost on history and clinical signs. Baseline bloodwork can reveal anemia (low red blood count), hypercholesterolemia (high cholesterol), and an abnormally low level of T4. However a low level of T4 hormone in itself is not diagnostic for hypothyroidism. It should prompt additional hormonal testing.  The most comprehensive and diagnostic testing profile includes T4, free T4 (ED), T3, TSH and antithyroglobulin antibodies. The Veterinary School at MSU (Michigan State University) and the OFA (orthopedic foundation for animals) are amongst the best diagnostic labs for hypothyroidism. The diagnosis however may still not be straight forward.  The hormones can vary hourly, non thyroidal illness may obscure the diagnosis and many medications (NSAIDS, anti-seizure and heart medications) may obscure the results. Given the limitations of testing some owners and veterinarians will perform a medication trail and see if the problems resolve with treatment.

Unlike testing, treatment is very straight forward. The deficient hormone is replaced by an oral thyroid hormone pill.  It is given either once or twice daily, is relatively inexpensive and has minimal side effects. If the dose is too high you may note increased thirst, weight loss, and anxiety as if your dog has had too much caffeine or RedBull. Thus it is important to have the blood checked every 6-12 month and important to note that it is not a cure; oral supplementation is life long. With treatment rapid improvement of activity level is seen but improvement of coat health may lag behind 4-6 months.