Signs of Hyperthyroidism in Dogs

The most common signs of hyperthyroidism in dogs are weight loss despite a good or increased appetite, drinking and urinating more, panting, restlessness, and a fast heart rate. Some dogs also become irritable or aggressive, have vomiting or loose, large stools, or seem weak (Heseltine, 2024; Köhler et al., 2012; Zeugswetter et al., 2013). A lump in the neck may be the first sign, or the only one, when a thyroid tumor is the cause (Prastiti et al., 2018).

Hyperthyroidism is rare in dogs, unlike in cats, and it has two main causes. One is a hormone-producing thyroid carcinoma, a type of cancer. The other is eating raw food, commercial food, or treats contaminated with animal thyroid tissue (Broome et al., 2015; Heseltine, 2024; Köhler et al., 2012). A dog being treated for hypothyroidism can also develop it if they're getting more thyroid medication than they need (Heseltine, 2024). Because a tumor is one possible cause, these signs deserve a prompt veterinary visit.

Key points

• Hyperthyroidism means the body has too much thyroid hormone, which speeds up metabolism. In dogs it is exceptionally rare (Zeugswetter et al., 2013).
• Thyroid carcinoma is the main cause in dogs, and a thyroid tumor that causes hyperthyroidism should be presumed malignant until proven otherwise (Heseltine, 2024).
• Most thyroid tumors in dogs do not cause hyperthyroidism. Only about 10% to 22% are reported to be hormone-producing (Prastiti et al., 2018).
• Diet-related hyperthyroidism is real and reversible. In two case series, thyroid hormone returned to normal and signs resolved after the contaminated food was stopped (Broome et al., 2015; Köhler et al., 2012).
• Several commercial dog treats and foods were recalled in 2018 because of elevated beef thyroid hormone (dvm360, 2018).
• Some dogs with high thyroid hormone show no signs at all. In one series, half of the affected dogs had no clinical signs (Köhler et al., 2012).

Detailed explanation

What hyperthyroidism is

The thyroid is a pair of small glands in the neck that make hormones controlling how fast the body burns energy. When there's too much hormone in circulation, the body runs too fast. The heart beats faster, calories are burned faster than they're replaced, and the dog may seem wired or uncomfortable.

Many people know hyperthyroidism as a cat disease. In dogs, it's exceptionally rare and typically results from thyroid carcinoma (Zeugswetter et al., 2013). Dogs being treated for an underactive thyroid are another group where it shows up (Heseltine, 2024).

The signs, one by one

Weight loss with a good appetite. This is the classic pattern: a dog who eats well, or even seems hungrier than usual, yet keeps losing weight (Heseltine, 2024; Prastiti et al., 2018). Weight loss was among the signs in dogs with diet-related hyperthyroidism (Köhler et al., 2012) and in dogs with functional thyroid carcinoma (Falcão, 2013). Not every dog loses weight, though; two dogs eating thyroid-contaminated meat in one report kept a good appetite and didn't lose weight (Zeugswetter et al., 2013). Our article on why dogs with cancer lose weight covers other reasons for this sign.

Drinking and urinating more. Increased thirst and urination appear on every major list of signs (Heseltine, 2024; Prastiti et al., 2018). In the two dogs described by Zeugswetter and colleagues (2013), gradually worsening thirst and urination were the most prominent signs. Many other conditions cause the same change, as our guide to increased thirst in dogs with cancer explains.

Panting and restlessness. Excessive panting and restlessness were reported in dogs with diet-related hyperthyroidism in two separate case series (Köhler et al., 2012; Zeugswetter et al., 2013). Owners may notice a dog that can't settle at night or pants in a cool room. See our article on panting in dogs with cancer for other causes.

Fast heart rate. Tachycardia, a heart rate faster than normal, was found in affected dogs (Köhler et al., 2012). One dog collapsed during a walk, and a fast heart rhythm was the only abnormality found (Zeugswetter et al., 2013). Any collapse needs immediate attention.

Behavior changes. Hyperactivity and aggressiveness have been reported (Heseltine, 2024; Köhler et al., 2012). A previously calm dog who becomes irritable or snappy for no clear reason is worth a checkup.

Digestive signs. Vomiting, large-volume stools, and other gastrointestinal problems can occur (Falcão, 2013; Heseltine, 2024).

A lump in the neck. Most thyroid tumors in dogs show up as a mass in the neck, usually below the windpipe. A large mass can also cause coughing, breathing changes, or regurgitation by pressing on the airway or esophagus (Prastiti et al., 2018). When a tumor is hormone-producing, signs of hyperthyroidism can appear before the mass is large enough to feel (Falcão, 2013).

Cause 1: a hormone-producing thyroid tumor

About 90% of thyroid tumors in dogs are carcinomas, and only about 10% to 22% of them produce excess hormone. Most affected dogs have normal thyroid levels, and some even become hypothyroid (Prastiti et al., 2018). So a normal thyroid blood test doesn't rule out a thyroid tumor, and a high one in a dog not eating a suspect diet raises real concern.

When a dog's hyperthyroidism comes from a tumor, veterinarians assume it's malignant until proven otherwise (Heseltine, 2024). Thyroid carcinomas usually spread to the lungs and nearby lymph nodes, and spread is already present in an estimated 16% to 38% of dogs at diagnosis, with larger tumors more likely to have spread (Prastiti et al., 2018). That's why your veterinarian may recommend chest X-rays and other staging tests once a thyroid mass is found. Our overview of thyroid cancer in dogs covers diagnosis and treatment options in depth.

Medication can bring hormone levels down, but it doesn't stop the tumor from growing or spreading, and the long-term outlook for dogs with hormone-producing thyroid carcinoma is described as poor to grave (Heseltine, 2024). A veterinary oncologist can help you weigh the treatment options for your dog's specific tumor.

Cause 2: thyroid tissue in food

Some dog foods and treats contain meat from the neck area of cattle or other animals, and if the thyroid gland isn't removed, the dog eats active thyroid hormone along with it.

In a German series of 12 dogs with high thyroid hormone and a history of raw feeding, six had signs consistent with hyperthyroidism and six had none. In all eight dogs rechecked after the diet changed, hormone levels returned to normal and signs resolved (Köhler et al., 2012). In Austria, two dogs fed raw head meat containing thyroid tissue developed increasing thirst, urination, panting, and restlessness; four of five other dogs fed meat from the same supplier also had high hormone levels, and levels normalized when the meat was removed or reduced (Zeugswetter et al., 2013).

This isn't limited to raw diets. In a U.S. series of 14 dogs, the source was all-meat commercial dog food or treats. Four weeks or more after those products were stopped, thyroid hormone was back in the normal range in every dog and any signs had resolved (Broome et al., 2015). In 2018, several commercial treat and food products were recalled for elevated beef thyroid hormone; coverage of one recall described increased thirst, urination, heart rate, weight loss, and restlessness as possible effects and noted that signs may resolve once the product is stopped (dvm360, 2018).

This is the most important practical point in the whole topic: if your dog has signs of hyperthyroidism, tell your veterinarian everything your dog eats, including raw diets, toppers, chews, and treats. A diet history can spare your dog an unnecessary cancer workup, and a diet change can be the entire treatment.

Cause 3: too much thyroid medication

Dogs with hypothyroidism take thyroid hormone replacement. If a dog is getting more than they need, they can develop signs of hyperthyroidism, and the fix is usually a medication adjustment by your veterinarian (Heseltine, 2024). If your dog is on thyroid medication and starts losing weight, panting, or drinking more, call your veterinarian rather than changing the amount yourself.

How it's diagnosed

Diagnosis starts with a blood test for thyroid hormone, usually alongside a full blood panel and urinalysis to look at the other causes of weight loss and thirst. If thyroid hormone is high, the next questions are what your dog eats, whether they take thyroid medication, and whether there's a mass in the neck. Thyroid scintigraphy, a nuclear imaging scan, can help distinguish the causes: dogs with diet-related disease showed decreased uptake in the thyroid gland on scans (Broome et al., 2015).

When to contact a veterinarian

Go to an emergency hospital right away if your dog collapses, faints, has trouble breathing, or has a very fast or irregular heartbeat.

Make a prompt appointment if your dog:

• Is losing weight while eating normally or more than usual
• Is drinking and urinating noticeably more
• Pants or paces at rest, especially at night
• Has a new lump anywhere in the neck, or a cough, voice change, or trouble swallowing
• Has become unusually irritable or aggressive
• Is on thyroid medication and shows any of these signs

Questions to ask your veterinarian

• Could my dog's diet, treats, or chews contain thyroid tissue, and should we change them before testing further?
• Does my dog need a thyroid blood test, and should it be repeated after a diet change?
• Do you feel any mass in my dog's neck?
• If there's a mass, would an aspirate, ultrasound, or chest X-rays be the next step?
• Could my dog's thyroid medication need adjusting?
• If this is a thyroid tumor, what are the treatment options, and should we see an oncologist?
• What other conditions could explain my dog's weight loss and thirst?

References

Broome, M. R., et al. (2015). Exogenous thyrotoxicosis in dogs attributable to consumption of all-meat commercial dog food or treats containing excessive thyroid hormone: 14 cases (2008-2013). Journal of the American Veterinary Medical Association, 246(1), 105-111. 

dvm360. (2018, March 23). Recall: Dog treats recalled due to elevated levels of thyroid hormone. https://www.dvm360.com/view/recall-dog-treats-recalled-due-to-elevated-levels-of-thyroid-hormone

Falcão, S. (2013). Hyperthyroidism in two dogs with a functional thyroid gland carcinoma [Conference proceedings]. Southern European Veterinary Conference, Barcelona. IVIS. https://www.ivis.org/library/sevc/sevc-annual-conference-barcelona-2013/hyperthyroidism-two-dogs-a-functional-thyroid-gland-carcinoma

Heseltine, J. (2024). Hyperthyroidism in animals. Merck Veterinary Manual, Professional Version. Retrieved November 5, 2026, from https://www.merckvetmanual.com/endocrine-system/the-thyroid-gland/hyperthyroidism-in-animals

Köhler, B., Stengel, C., & Neiger, R. (2012). Dietary hyperthyroidism in dogs. Journal of Small Animal Practice, 53(3), 182-184. 

Prastiti, E., Tzenetidou, Z., & Papazoglou, L. G. (2018). Canine thyroid tumours: Diagnosis and treatment. Hellenic Journal of Companion Animal Medicine, 7(2), 19-27. https://www.hjcam.hcavs.gr/images/vol7iss2/izs_v7i2-canine-hyroid-en.pdf

Zeugswetter, F. K., Vogelsinger, K., & Handl, S. (2013). Hyperthyroidism in dogs caused by consumption of thyroid-containing head meat. Schweizer Archiv für Tierheilkunde, 155(2), 149-152.