Thyroid disorders are the most common hormone imbalances that we see in dogs and cats. Dogs tend to have under functioning thyroids (hypothyroidism) and cats usually have thyroids that overproduce thyroid hormones (hyperthyroidism).
Hyperthyroidism
This disease occurs most commonly in older cats (over 10 years of age) and the hallmark sign is weight loss despite having a normal to increased appetite. Other signs that are commonly seen include; restlessness, change in behavior, poor haircoat, increased drinking and urination, or occasional vomiting. On physical examination, these cats will typically have an increased heart rate, are underweight, and sometimes the enlarged thyroid glands can actually be palpated. A simple bloodtest can be done to measure the thyroid levels along with a complete blood panel and urinalysis. In most cases, the thyroid levels will come back very elevated meaning a diagnosis of hyperthyroidism can be made, but sometimes the values are normal or only mildly increased so additional testing may be necessary.
Commonly, cats can have concurrent disease with hyperthyroidism. Heart disease as well as high blood pressure and possibly mild heart failure are usually reversible once the disease is treated. Mild elevations in the liver and kidney enzymes are also seen. Once treatment is initiated, underlying kidney disease may worsen, so rechecking blood values is very important. If your cat has renal dysfunction and hyperthyroidism, the short-term prognosis is good, but the long-term prognosis is poor because they are both progressive illnesses and treatments for either will worsen the other disease.
Treatment consists of three main categories: lifelong treatment with diet and/or medication, surgical removal of the thyroid glands, or treatment of the glands with radioactive iodine. Methimazole is the oral medication approved for treating hyperthyroidism. It needs to be given twice daily for the life of the pet. Hills science diet recently introduced a new food named y/d that has been shown to improve thyroid health within 3 weeks when feed as the only food source. Surgical removal is done at a referral hospital and is generally permanent with no medications needing to be given after surgery. In rare cases, there can be thyroid tissue located somewhere else in the body that will continue to produce thyroid hormones and so removal will not be effective. Lastly, radioactive iodine treatment is very effective and curative - meaning no long term medication. Also, the pet does not have to undergo anesthesia or invasive surgery. The downside is that it is only offered at special facilities (Texas A&M, Gulf Coast Veterinary Specialists) and the expense is more. The long-term prognosis for surgery and radiotherapy is good (barring no other health problems). Recommended monitoring includes checking the thyroid levels 2 times a year.
Hypothyroidism
This disease usually occurs in middle-aged to older dogs with definite disease predisposition to Doberman Pinchers, Golden Retrievers, Irish Setters, Great Danes, Dachshunds, Poodles, and Boxers. Signs are usually nonspecific and gradual in onset, and include; weight gain, lethargy, mental dullness, exercise/cold intolerance, and a thin haircoat with a greasy coat and secondary skin infections. On physical exam, these dogs may have a slower than normal heart rate, are obese, and many have some type of dermatologic issue. Testing for hypothyroidism can be more difficult than for diagnosing hyperthyroidism. Usually several thyroid hormone levels need to be evaluated for a confirmative diagnosis. A low T4 (total T4) can occur with other diseases, secondary to certain medications, or in breeds like Greyhounds. If the T4 comes back as low, a freeT4 and TSH levels will need to be evaluated to see if the thyroid is truly low due to disease. As well as checking thyroid levels, a complete blood count and urinalysis should be performed. Common changes include a high cholesterol, mild anemia, and mild increases in the liver enzymes.
Once a diagnosis is made, treatment is quite simple. The dog will need to be given levothyroxine (an oral thyroid supplement) twice daily. This medication is relatively inexpensive but must be continued for the life of the animal. Commonly, secondary skin infections may need to be treated, and improvement in skin/coat abnormalities should be seen 1-4 weeks after starting the thyroid supplement. The long-term prognosis is good to excellent for a return to function. Recommended monitoring includes checking the thyroid levels 1-2 times a year.
-Dr. Tricia Trunek, DVM
