Guide to Pet Medications: Thyroid Medications and Your Pet

In dogs and cats, the two thyroid glands sit in the middle of the neck on either side of the trachea. These glands produce thyroid hormone, which is necessary to maintain a healthy metabolism. If the thyroid gland malfunctions, multiple organ systems are affected and the pet can become very sick. This can happen with either too much (hyperthyroidism) or too little (hypothyroidism) thyroid hormone production. Hyperthyroidism affects cats while hypothyroidism affects dogs. Both diseases are very common in veterinary medicine.

There are two different types of thyroid medication: there is a supplement for dogs that make too little thyroid hormone and there is a medication that blocks production for those cats that make too much.

Thyroid Supplementation for Hypothyroid Dogs

Hypothyroidism involves a deficiency of thyroid hormone production and is usually caused by immune-mediated destruction of the thyroid tissue (the dog’s own body attacks the thyroid gland). This hormone deficiency can cause skin disease, hair loss, obesity, infections, lethargy, cardiovascular signs, or neurologic disorders.

The thyroid produces active hormone and inactive hormone.  The inactive hormone circulates in the blood until it is needed and is then converted to the active form.  Diagnosis can be challenging because the standard blood test measures the inactive form only, and because thyroid levels will be secondarily decreased in the presence of any illness. In some cases, the doctor may need to perform multiple blood tests to be certain that the dog is truly hypothyroid. 

After the diagnosis is confirmed, your veterinarian will prescribe a pill containing the inactive hormone (thyroxine) and then your dog will convert it to the active version. 

Treatment for Hyperthyroid Cats

Hyperthyroidism involves an overproduction of thyroid hormone due to an over-functioning thyroid gland. This overproduction is toxic to the heart muscle and sends the metabolism into overdrive. We do not yet know why the thyroid begins to over-produce. These patients typically demonstrate a ravenous appetite while losing weight. There may be other clinical signs, such as urinating large volumes of urine, skin disease or hair loss, or cardiac disease. 

The diagnosis is made based on blood values, the presence of a palpable thyroid nodule (when the gland is large enough to feel from the outside of the cat’s neck), and the presence of clinical signs. All older cats should have their thyroid levels checked every 6-12 months, because this disease is very common.

Your veterinarian will prescribe a pill containing methimazole, which blocks the production of both active and inactive hormone. The cat’s metabolism can return to normal, and she will begin to gain weight again.

What happens next?

1. After the diagnosis, your pet will begin a course of medication at the initial dose.   
2. Your veterinarian will then repeat the blood work a short time later to determine if your pet is on the right dose.  Sometimes the blood needs to be rechecked several times initially, usually once per month, in order to titrate the proper medication dose.   
3. Once your pet is regulated and the thyroid levels are normal, your veterinarian will likely want to recheck the lab work every 3-6 months.   

What are the side-effects of thyroid medications?

Thyroid supplementation for hypothyroid dogs is very safe and side-effects are rare. Consult your veterinarian if your dog is taking any other medication (including seizure medicines or prednisone (steroids)) because these can affect thyroid levels and might make him difficult to regulate.

Methimazole administration for cats is relatively safe, but some may experience an adverse reaction to the drug.  The most common reactions are gastrointestinal, such as decreased appetite and vomiting, but these clinical signs appear to be temporary and mild. There can also be severe dermatologic side-effects. These cats will itch and scratch at their faces and the medication will have to be discontinued. Rarely, a cat may experience liver disease or bone marrow abnormalities.  Methimazole can be taken in pill form or compounded into a flavored treat, flavored liquid, or into a transdermal gel that is applied to the hairless portion of the inner ear flap. The transdermal gel appears to cause fewer gastrointestinal adverse effects than the oral versions.

Hyperthyroidism has a protective effect on the kidneys and might mask pre-existing kidney disease. Correcting the thyroid levels unmasks the disease and may cause a cat to decompensate into renal failure. In these cases, the veterinarian might choose to keep the cat on a low dose of drug even if the thyroid disease is not completely under control.

Are there alternatives to methimazole?

The “gold standard” treatment for hyperthyroid cats today is irradiation of the thyroid gland with radioactive iodine. This treatment involves a short stay at a specialty hospital but is usually curative. It is expensive but might be more cost-effective than methimazole in the long run.

Another option is surgical thyroidectomy to remove the overactive gland.  Finally, for those cats who can’t tolerate methimazole’s side effects, there are alternative medications such as ipodate or ipanoic acid.  These medications are rarely prescribed and are difficult to locate.