Exogenous Hyperthyroidism and Thyroid Hormones in Pet Food - Veterinarians
March 27, 2017
The FDA’s Center for Veterinary Medicine has recently investigated a cluster of cases with three dogs of various ages, in separate households, that exhibited symptoms of hyperthyroidism. Hyperthyroidism in dogs is in itself unusual; however, the unique presentation of hyperthyroidism symptoms in these dogs is especially noteworthy for practitioners.
The three dogs had extensive testing at a reference laboratory. Results showed elevated thyroid hormone in the blood. Dietary history interviews conducted by the reference lab indicated that they all ate canned dog food from at least one of two companies. After consultation with the FDA, those companies recalled the implicated lots of the two dog foods that had been fed to the affected dogs. The company-issued press releases, including recalled product information, are available using the hyperlinks below:
Three dogs of different ages, sexes, and breeds, including a 4-year-old Shetland Sheepdog, 8-year-old Tibetan Terrier, and 15-year-old Labrador Retriever, presented with various clinical signs of hyperthyroidism such as increased thirst (polydipsia), increased urination (polyuria), restless behavior, and weight loss.
A full thyroid panel for T3, free T3, T4, free T4, TSH, thyroid autoantibodies, and iodine were conducted at a reference laboratory.
Results from all three dogs on this full thyroid panel are detailed below. Values and ranges are not included as there are variations between laboratories.
- active (T3) and inactive forms (T4) of thyroid hormones
- HIGH total T3
- LOW total T4
- protein bound and unbound forms (free T3, free T4) of thyroid hormones
- HIGH free T3
- LOW free T4
- thyroid stimulating hormone (TSH)
- low NORMAL TSH
- autoantibodies (T3, T4, thyroglobulin)
- NORMAL autoantibodies (T3, T4, and thyroglobulin)
- total iodine
- elevated total iodine
Based on the pattern, the reference lab’s consulting veterinarian suspected an exogenous source of thyroid hormones stemming from dietary exposure and recommended modifying the dogs’ diets from the canned foods to a different dog food for several weeks to determine whether the canned foods were associated with the hyperthyroid condition.
The dogs were then retested with a full thyroid panel. After three to four weeks on a different food, two of the dogs’ clinical signs improved and thyroid hormones normalized. The other dog’s clinical signs also improved, but the dog had low total T4, free T4, and total T3. An additional panel was not obtained.
Please note that in some historical cases of exogenous hyperthyroidism, the pattern of the thyroid panel results showed differences in the elevated hormones (e.g., increased T4) than these three cases. Given the limited number of reported cases, there may be a spectrum of possible results in the thyroid panel that could be associated with consumption of dog food containing thyroid hormones.
Based on the improved health of all three dogs after eating different food, their original canned dog foods were investigated by FDA as possible exogenous sources of thyroid hormones. Interviews conducted by the reference lab found a shared exposure to certain canned dogs foods. The FDA collected samples of these dog foods, which included BLUE Wilderness® Rocky Mountain RecipeTM Red Meat Dinner Wet Food for Adult Dogs and Wellness 95% Beef Topper for Dogs. The samples were analyzed by the FDA’s Forensic Chemistry Center (FCC) for thyroid hormone concentrations using validated and published analytical methods to detect iodine species. Iodine species include the precursors, 3-moniiodotyrosine (MIT) and 3,5-diiodoytrosine (DIT) that form T3 and T4. The BLUE Wilderness® Rocky Mountain RecipeTM Red Meat Dinner Wet Food for Adult Dogs and Wellness 95% Beef Topper for Dogs samples tested positive for:
- thyroid hormone T3
- iodine species 3-monoiodotyrosine (MIT)
- iodine species 3,5-diiodotyrosine (DIT)
The presence of thyroid hormone and iodine species in the canned dog foods confirmed that the food was an exogenous source of thyroid hormones. This exogenous source of thyroid hormones was capable of causing clinical signs and abnormal thyroid hormone panel results in the affected dogs.
What You Need to Know
This cluster of cases and the unique presentation of clinical signs of hyperthyroidism reinforce the importance of practitioners getting a detailed dietary history, including the type and amount of food as well as frequency of consumption. If a hyperthyroid dog tests negative for thyroid cancer, this information can help identify possible dietary exogenous sources of thyroid hormones. In addition, practitioners who suspect that a dog may have exogenous hyperthyroidism may want to run a full thyroid panel including T3, free T3, T4, free T4, TSH, thyroid autoantibodies, and iodine.
If you encounter a case with lab results and symptoms consistent with exogenous hyperthyroidism, please report it to the FDA.
Additional Background on Thyroid Gland in Pet Foods
Animal food with elevated thyroid levels likely contains animal gullets (laryngeal tissue) in which the thyroid glands were not completely removed.
Human cases of exogenous thyrotoxicosis have been reported after consumption of cattle and lamb products containing gullet.1 USDA prohibits the use of thyroid glands and laryngeal muscle tissue for human food.
Sporadic cases of exogenous thyrotoxicosis in dogs have been reported, and subsequent testing of the food revealed detectable thyroid hormones.2 Laryngeal tissue (gullets) obtained from beef and lamb slaughter establishments used in the manufacture of pet treats and pet food could be a potential source of thyroid tissue. If a thyroid gland is not completely removed from a gullet and that gullet is then added to pet food or treats, remnant thyroid tissue could be a source of thyroid hormones.
1 Hedberg, eW, An outbreak of thyrotoxicosis caused by the consumption of bovine thyroid gland in ground beef. New Enl J Med 316: 993-998.
2 Broome, MR, 2015. Exogenous thyrotoxicosis in dogs attributable to consumption of all-meat commercial dog food or treats containing excessive thyroid hormone: 14 cases (2008-2013). J Am Vet Med Assoc 246: 105-111.