Canine Hypothyroidism

            Hypothyroidism is the most common hormonal imbalance in dogs and represents a deficiency of thyroxine (tetraiodothyronine – T4 and triiodothyronine – T3).  Thyroxine is a hormone produced by the thyroid gland in the neck and affects cellular metabolism in most of the tissues of the body.  Hypothyroidism is uncommon in cats.  In dogs, hypothyroidism can be either primary or secondary in nature.  Primary hypothyroidism (more than 95% of all cases) is usually caused by immune-mediated destruction or inflammation of the thyroid gland (lymphocytic thyroiditis) or replacement of thyroid tissue with fat cells for unknown reasons (thyroid atrophy).  Uncommon causes of primary hypothyroidism include dietary iodine deficiency, cancer, infection, and surgical removal of the thyroid gland.  Secondary hypothyroidism (less than 5% of all cases) is caused by reduced secretion of thyroid-stimulating hormone (TSH) from the pituitary gland at the base of the brain due to congenital malformation, cancer, or infection.  TSH production can also be affected by glucocorticoids (steroids), concurrent illness, and malnutrition.

                The clinical signs of canine hypothyroidism typically develop gradually, affect a number of different organ systems, and range from mild to severe.  Clinical signs are summarized below by body system.

General Appearance / Behavior                               Skin / Hair                             

            weight gain                                                                   excessive scale and dry hair coat          

            lethargy, dullness                                                           excessive shedding                               

cold intolerance, heat-seeking                                       hair loss, usually symmetrical                

            exercise intolerance                                                       increased pigmentation of the skin

                                                                                                recurrent skin/ear infections

Cardiovascular System                                                          thickening of the skin    

            slow heartbeat                                                                         

            weak pulses                                                      Eyes

            arrhythmias (rare)                                                         corneal lipid deposits

                                                                                                chronic uveitis

Nervous System / Muscle                                        

            peripheral neuropathy                                       Reproductive System

            megaesophagus (rare)                                                   anestrus, infertility, and abortion

            nonspecific weakness                                                    inappropriate milk production

            behavioral changes                                                        testicular atrophy

DIAGNOSIS

            The diagnosis of hypothyroidism in dogs is based on a thorough history including current medications, diet, and lifestyle, a physical examination to assess clinical signs, and laboratory tests.  Routine laboratory tests (complete blood count, chemistry profile, urinalysis) may be run to evaluate for non-thyroid illness.  Additionally, there are a variety of tests available to assess for thyroid disease.  The most reliable of these thyroid-specific tests include serum total T4, free T4 by equilibrium dialysis, and serum endogenous TSH.  Dogs with hypothyroidism usually have decreased T4 and elevated TSH.  Unfortunately, it is possible for a dog to have abnormal thyroid values but not be truly hypothyroid due to a variety of other conditions (euthyroid sick syndrome).  If clinical signs and laboratory testing support a diagnosis of hypothyroidism, then a trial period of thyroid supplementation will be started, and response to therapy will be evaluated. 
 

TREATMENT

            Dogs suffering from primary hypothyroidism are generally easy to treat and respond well to therapy.  The treatment of choice is synthetic levothyroxine (L-thyroxine).  The medication may be given once or twice daily depending on the individual animal.  Individual dose requirements and dose frequency may change over time.  Dogs generally respond to thyroid supplementation gradually.  Energy level and attitude often improve within seven to ten days.  Weight loss and skin and hair coat problems may take several weeks to resolve.  A period of at least two to three months is recommended to determine the effectiveness of treatment.  Please remember that thyroid supplementation is required for the life of your dog.  If you discontinue giving the medication, clinical signs of the disease will return.  Periodic rechecks of your dog’s thyroid level (T4) will be needed to determine the proper dose of medication.  These rechecks are usually scheduled six hours after giving the medication when T4 levels should be near the upper limit or slightly above the reference range.  Over dosage of L-thyroxine results in signs of hyperthyroidism including anxiety, increased drinking and urination, weight loss, vomiting, diarrhea, and increased heart rate.  Please notify us if you suspect that your dog is being over dosed.     

If your dog’s hypothyroidism is believed to be due to non-thyroid illness we will likely want to treat the underlying illness and monitor for resolution of the hypothyroidism.

PROGNOSIS

            Dogs suffering from primary hypothyroidism that are properly supplemented with thyroid hormone have a good prognosis and their life expectancy is likely to be normal.  However, dogs with secondary hypothyroidism generally have a guarded to poor prognosis given the likelihood of destructive, expanding lesions in the brain.

            If you ever have questions regarding any of the above information, please do not hesitate to contact us.  Visit us online at www.WhiteBearAnimalHospital.com.

WHITE BEAR ANIMAL HOSPITAL                           OTTER LAKE ANIMAL CARE CENTER

          1909 East County Road E                                                     6848 Otter Lake Road

       White Bear Lake, MN 55110                                                      Hugo, MN 55038

                  (651) 777-1393                                                                 (651) 426-8871