HYPOTHYROIDISM IN DOGS
Hypothyroidism develops from a decrease in blood levels of thyroid hormone. It is perhaps the most common hormonal problem we encounter in dogs. Because thyroid hormone is involved in maintaining normal metabolism, activity levels, hair growth, reproduction, heart function, nervous system function, and most other bodily functions, low levels of thyroid hormone can produce a wide variety of signs of illness. Some of these signs are obvious, and some are very subtle.
The most common type of hypothyroidism, called primary hypothyroidism, is due to inflammation that leads to scarring and destruction of the thyroid glands. This inflammation arises spontaneously when the pet's immune system behaves abnormally by attacking the thyroid gland; there typically is no detectable reason why the immune system does this. Less commonly, infection, tumors and trauma can damage the thyroid glands and lead to primary hypothyroidism. Even less commonly it may be congenital.
Hypothyroidism is inherited in some dog breeds, including: Airedales, Beagles, Boxers, Dachsunds, Dobermans, Bulldogs, Golden Retrievers, Great Danes, Irish Setters, Newfoundlands, Poodles, and Shelties.
Hypothyroidism can also develop with certain types of drug use, surgery of the neck area (where the thyroid glands are), and radiation therapy.
Some dogs that are significantly ill with non-thyroid diseases have low levels of thyroid hormone. This is a normal metabolic response to illness, these patients are not truly hypothyroid, and they do not require treatment for it.
The signs of hypothyroidism are usually slow to develop and may be obvious or subtle. They can include one or more of the following:
- Skin: fur loss, poor quality fur, excess dandruff and dry skin
- Metabolism: lethargy, seeking warmth, weight gain, obesity, inability to exercise
- Heart: low heart rate, hardening of arteries
- Nervous system: muscle weakness, muscle shrinkage, paralysis of larynx (vocal cords), paralysis of other nerves of the head
- Reproductive: infertility, failure of heat cycles
- Cretinism: (very uncommon in dogs) mental and growth retardation, retention of puppy coat, overly large head, skeletal dwarfism
We diagnose hypothyroidism by measuring low levels of thyroid hormone in the blood. It is sometimes necessary to run more than one type of thyroid blood test and it is sometimes necessary to repeat the thyroid tests to confirm the diagnosis.
When we are first attempting to diagnose the problem we take a complete patient history, perform a thorough physical exam, and run a general blood profile, a urinalysis, and sometimes other tests in addition to the thyroid tests, because of the importance of identifying all of the problems the patient might have.
Hypothyroidism is treated with a daily oral thyroid hormone supplement. Most dogs require twice daily dosage for life. Possible side effects include increased thirst, urination and appetite, panting, and hyperactivity. Side effects are easily relieved by simply lowering the dose of the supplement.
FOLLOW-UP / MONITORING
Four to six weeks after the start of treatment we repeat the thyroid blood tests (and sometimes the other tests as well) to help us determine if the pet's dose of thyroid supplement needs to be adjusted. Periodic retesting (usually once yearly) is needed throughout the dog's life to determine the need for additional dose adjustments.
Signs improve within 3 to 4 weeks, except for nervous system signs, which can take several months. Exercise and activity levels improve noticably and appetite returns to normal. Skin and fur problems resolve. Most dogs lead normal lives.
Yarmouth Veterinary Center