What Is It?
Hypothyroidism is an endocrine disorder in which the thyroid gland does not produce the normal amount of thyroid hormones. Primary hypothyroidism is the most common form of this disorder in dogs and is a disorder at the level of the thyroid gland itself. Most often, primary hypothyroidism is due to lymphocytic thyroiditis (inflammation of the thyroid gland and infiltration of the gland with immune cells) or idiopathic thyroid atrophy (loss of normal thyroid tissue and replacement by fat). Rarely, hypothyroidism is due to a disorder of the pituitary gland (secondary hypothyroidism) or the hypothalamus (tertiary hypothyroidism). Hypothyroidism may also be due to a congenital disorder or secondary to treatment with some medications.
Generally, hypothyroidism affects dogs between the ages of 2 and 6 years. Since the thyroid hormones are responsible for metabolism, the most common clinical signs include lethargy, mental dullness, inactivity, weight gain without an increase in appetite, and cold intolerance. Since the onset is often gradual, the owner may not even notice these signs. Dogs with this disease often present to the veterinarian for skin conditions. Bilateral hair loss on the trunk or tail (“rat tail”), a dry brittle hair coat, flaky skin, secondary bacterial infections, and ear infections are the most common complaints. In addition, dogs with hypothyroidism often accumulate fluid in their skin causing the skin to thicken, especially in the forehead, face, and eyelids. Because this makes the dog look very sad, this is referred to as the “tragic facial expression”. Dogs with hypothyroidism may present for neurological abnormalities, and occasionally, these neurological problems are the only complaint. These abnormalities may include seizures, ataxia (difficulty walking), circling, facial nerve paralysis, weakness, and dragging the feet. Other clinical signs that may be associated with hypothyroidism include reproductive abnormalities, ocular (eye) abnormalities, bradycardia (slower heart rate), diarrhea or constipation, bleeding disorders, and occasionally behavioral abnormalities.
A thorough evaluation is necessary whenever hypothyroidism is suspected. This will include a thorough medical history, a complete physical examination, a complete blood count (CBC), a serum chemistry profile, and a urinalysis (UA).
Confirmation of the diagnosis of hypothyroidism depends on specific testing of the hypothalamic-pituitary-thyroid gland axis. A number of hormones can be measured to obtain a diagnosis of hypothyroidism, but the current recommendations for the assessment of thyroid gland function is to measure the serum total T4, the serum fT4, and the serum endogenous TSH concentration in combination. In practice, however, the diagnosis of hypothyroidism often relies solely on the measurement of serum total T4 along with the appropriate history, clinical signs, and preliminary blood work results.
T4 or thyroxine is a circulating thyroid hormone, and measurement of the serum total T4 measures both the protein-bound and the free hormone in circulation. This is an excellent screening test. If it is normal and your dog is not showing classic clinical signs of this disease, hypothyroidism can be ruled out. If it is below normal, especially greatly below normal, and your dog has the typical history and clinical signs of hypothyroidism, a tentative diagnosis can be made. However, if this measurement is below normal and clinical signs are questionable, your dog may not necessarily have hypothyroidism as concurrent illness, medications, and random daily fluctuations in thyroid hormone can cause a falsely lowered total T4 value. Likewise, while less common, a dog with hypothyroidism may have a falsely normal T4 value. If this test is inconclusive, it is best to perform the other two tests mentioned above.
The serum fT4 test measures only the free (not protein-bound) thyroid hormone in circulation. This is less likely to be lowered by medications or concurrent illness, though it certainly can be. Finally, the serum endogenous TSH test, along with the previous two, will allow a definitive diagnosis in almost all cases. TSH is thyroid stimulating hormone. It is a hormone produced by the pituitary gland which serves to stimulate the thyroid gland to produce thyroid hormones. In most cases of hypothyroidism, this hormone will be elevated as it is attempting to stimulate the thyroid gland to correct the low levels of thyroid hormone in circulation. It does not realize that there is an intrinsic problem with the thyroid gland such that it is unable to respond (as in the case of primary hypothyroidism). The combination of decreased serum total T4 and fT4 and an elevated endogenous TSH allows the definitive diagnosis of hypothyroidism.
Numerous other tests are available, but are not often performed in dogs. Occasionally, testing for antibodies against thyroid hormones or a component of the thyroid gland itself (thyroglobulin) is performed to differentiate lymphocytic thyroiditis from idiopathic thyroid atrophy.
The treatment for hypothyroidism is the administration of the synthetic thyroid hormone, sodium levothyroxine (thyroxine). This medication should be given as directed for a minimum of 6-8 weeks before its effectiveness is evaluated. You should see a gradual increase in mental alertness, activity, muscle strength, and appetite, as well as an improvement in the haircoat and any neuromuscular or reproductive abnormalities that may have been present. After 4-6 weeks of treatment, the serum T4 should be measured to assess the response to treatment. Your veterinarian will take 1-2 blood samples for this purpose. Depending on the results, either the dose or the frequency of administration of the sodium levothyroxine may need to be altered. Any time an alteration is made, the serum T4 levels should be reevaluated after 2-4 weeks on the new dosing schedule. Once stabilized, the T4 levels should be measured every 6 months to 1 year.
Side effects of the sodium levothyroxine are rare but may include panting, nervousness, aggression, increased urination, increased water consumption, increased appetite, and weight loss. If any of these signs are detected, the serum T4 should be measured and the dose and/or frequency of medication adjusted as necessary.
Prognosis for dogs with primary hypothyroidism receiving the appropriate medication is excellent and the life expectancy is normal. For puppies with hypothyroidism, the prognosis is guarded and depends upon the severity of any skeletal and joint abnormalities. For dogs suffering from secondary or tertiary hypothyroidism, the prognosis is guarded to poor and depends on the cause of dysfunction.
If you have any questions or concerns regarding hypothyroidism or the medication used to treat it, please consult your veterinarian.