The treatment landscape for hypothyroidism in dogs is dramatically changing… but first some background on the disease for those of you who are unfamiliar with it.


Hypothyroidism is one of the most common endocrine (hormonal) diseases of dogs. The condition usually develops when the dog’s own immune system destroys functional thyroid tissue, resulting in lower than normal concentrations of thyroid hormone in the body.


The thyroid gland is essentially responsible for setting a dog’s metabolic rate, and some of the classic signs of hypothyroidism, such as weight gain, lethargy, and heat-seeking behaviors, reflect that role. Other common symptoms include recurrent infections (particularly of the skin and urinary tract) and hair loss. In some cases, seizures or other neurologic problems, tendon or ligament injuries, and a thickening of the skin producing a “tragic” facial expression may also develop.


When dogs have some of these symptoms and blood work has revealed low thyroid hormone levels, and they have not been diagnosed with another disease or treated with a drug known to reduce thyroid hormone levels, a tentative diagnosis of hypothyroidism is appropriate. I say “tentative” because the last stage of diagnosis should be response to treatment.


If your dog’s symptoms improve with thyroid hormone replacement therapy after recheck blood work has confirmed that therapeutic levels have been reached, you can be confident that your dog truly has hypothyroidism and that hormone replacement therapy should continue.


But now there are far fewer treatment options for hypothyroidism in dogs. Veterinarians in the United States used to have 10 brands of thyroid hormone replacement to choose from… now we have only one. True, all of these products contain the same active ingredient, levothyroxine, but most veterinarians can tell stories about how, for unknown reasons, Brand A seemed to work better for Boomer while Brand B was the better choice for Annie.


How did this happen? The U.S. Food and Drug Administration (FDA) recently approved one product, Thyro-Tabs Canine, for the treatment of hypothyroidism in dogs. Now that there is an FDA-approved drug available (none of the manufacturers had gone through this process in the past) it is illegal for other companies to manufacture or distribute levothyroxine for dogs. As the FDA announcement of the change put it:


In January 2016, FDA issued warning letters to companies manufacturing an unapproved levothyroxine product informing them that they are in violation of the law. If a company continues to manufacture an unapproved levothyroxine product, the agency may take enforcement action, such as seizing the illegal product, filing for an injunction to prevent further sale of the product, or both.


Unapproved animal drugs may not meet the agency’s strict standards for safety and effectiveness. They also may not be properly manufactured or labeled.


I don’t know whether this change will ultimately be beneficial or detrimental. Perhaps some of the variation veterinarians have seen in their patients’ response to levothyroxine has been due to inconsistent product quality, which shouldn’t be a problem in an FDA-approved drug. On the other hand, I can envision levothyroxine shortages and increased costs now that only one manufacturer is responsible for supplying the drug to all hypothyroid dogs in the U.S., at least until another company applies for FDA approval.


Only time will tell.