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Hyperthyroidism in dogs: high T4, raw diets and thyroid tumors

An overactive thyroid is common in older cats but rare in dogs. When a dog’s T4 comes back high, the cause is usually one of two things: a thyroid tumor that makes hormone, or thyroid hormone coming from outside, in food, treats or tablets. Here is how vets tell them apart.

Written by the AI DiagAnimals editorial teamReviewed by the AI DiagAnimals TeamUpdated Our editorial policy

The essentials

A high total T4 in a dog has two main explanations. A thyroid tumor can make too much hormone, but most dogs with a thyroid tumor have normal or low T4. Or the hormone comes from outside: raw diets, gullets or treats containing thyroid tissue, or too much levothyroxine. In diet-induced cases the TSH is very low and T4 returned to normal in all dogs once the food was changed. The diet history, TSH, a neck exam and scintigraphy point to the cause.

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What hyperthyroidism means in a dog

Hyperthyroidism means too much thyroid hormone (T4 and T3) in the blood. In dogs, the thyroid itself rarely becomes overactive on its own, as it does in cats. Thyroid cancers make up 1.1% of all tumors recorded in dogs, and 90% of them are carcinomas [1]. Only a minority make extra hormone: 8.7% of 103 tested dogs in one surgical study, and 29% of dogs in a large scintigraphy series [2, 3].

The other cause is hormone taken in from outside. Dogs fed raw meat, fresh or dried gullets, head meat or some all-meat foods and treats containing thyroid tissue can develop a high T4 [6, 7, 8]. So can dogs given too much levothyroxine, the tablet used for an underactive thyroid [9].

Causes of a high T4 and what the tests show

CauseWhat the tests showKey clue
Functional thyroid tumorHigh T4; the tumor takes up the tracer on scintigraphy [3]Often a mass in the neck of an older dog [1]; 85% of these tumors were malignant in one study [4]
Raw diet, gullets, head meat or treats with thyroid tissueHigh T4 with a very low TSH; small thyroid glands on ultrasound; low tracer uptake on scintigraphy [6, 7, 8]The diet history. T4 returns to normal after the food is changed [6, 7]
Too much levothyroxineHigh total and free T4, suppressed TSH [9]A dog on thyroid tablets, or one that eats the feces of a treated housemate [14]
Food rich in T3 (rare)High T3 while T4, free T4 and TSH are normal or low [12]Signs of an overactive thyroid that do not match the T4 [12]

Signs of hyperthyroidism in dogs

  • Drinking and urinating more: the most common sign (56%) in 27 dogs with a functional thyroid tumor [4].
  • Weight loss: in 44% of the same dogs [4], and in dogs with diet-induced cases [6].
  • Restlessness, aggressiveness, panting and a fast heart rate in dogs fed raw diets [6].
  • No signs at all: half of the 12 raw-fed dogs had none, and some tumors are found only on routine blood work [4, 6].

Increased thirst has many other causes, such as kidney disease, so a urinalysis and a CBC with chemistry are usually part of the workup.

The thyroid tests

TestIn hyperthyroidismWhat to know
Total T4 (TT4)HighMedian 8.8 µg/dL in 14 dogs fed food or treats with thyroid hormone [7].
Free T4 (fT4)High with excess levothyroxineRises along with total T4 when a dog gets too much hormone [9].
TSHVery low (suppressed)Below 0.03 ng/mL in 5 of 6 raw-fed dogs tested [6].
T3Sometimes highWorth measuring when signs do not match the T4 results [12].
Thyroid scintigraphyDepends on the causeLow uptake in diet-induced cases; uptake in the tumor when one is present [3, 7].

In a dog on levothyroxine, timing matters. Total T4 peaks about 5 hours after the tablet: in one study it averaged 23 nmol/L before the dose and 56 nmol/L at the peak [10]. A high result taken at the peak means something different from the same result taken before the dose.

How to read the T4 line

Here is an example line from a dog with a high T4.

An annotated line from a lab report · Illustrative example
AbbreviationResultUnitLaboratory intervalFlag
TT4 Total thyroxine6.5µg/dL1.0–4.0H
  1. Abbreviation TT4, T4 or thyroxine. Free T4 appears as fT4.
  2. Result 6.5 µg/dL in this illustrative line: clearly above the interval.
  3. Unit µg/dL in the United States, nmol/L in Europe (µg/dL × 12.87 = nmol/L).
  4. Laboratory interval An illustrative dog interval; each laboratory sets its own.
  5. Flag H: above the interval. Your vet asks about diet, treats and thyroid tablets, then checks TSH and the neck [6, 7].

How it differs from cats and from hypothyroidism

In cats, hyperthyroidism is the most common hormonal disease of older age, affecting up to 10% of cats over 10, and only 2% have a thyroid cancer at diagnosis [13]. See hyperthyroidism in cats. In dogs, it is the opposite: the disease is rare, and when a tumor is the cause it is usually malignant [1, 4].

Hypothyroidism, an underactive thyroid, is far more common in dogs. Most dogs with a thyroid tumor are not hyperthyroid at all: about 50–60% have normal thyroid values and 30–40% are hypothyroid [2]. See hypothyroidism in dogs.

Outlook

Diet-induced cases have a good outlook. In 14 dogs, T4 was back in the normal range and signs had resolved at least 4 weeks after the suspect food or treats were stopped [7]. In 8 raw-fed dogs rechecked after a diet change, T4 normalised in all [6].

For functional thyroid tumors, median survival was 35.1 months overall in 70 dogs, 72.6 months after surgery and 15.7 months without surgery [5]. After surgery, 64% of dogs developed an underactive thyroid [5].

Related articles

Hypothyroidism in dogs

T4, free T4 and TSH when the thyroid is underactive.

Read the article

Hyperthyroidism in cats

The common feline form, and its blood work.

Read the article

Kidney disease in dogs

Another cause of drinking and urinating more.

Read the article

See also the breed pages: Golden Retriever, Beagle, Siberian Husky, Labrador Retriever.

En français : l’hyperthyroïdie du chien.

Questions to ask your veterinarian

  1. Could my dog’s diet, chews or treats explain the high T4?
  2. Should we measure TSH, free T4 or T3 as well?
  3. If my dog takes levothyroxine, when was the sample taken compared with the tablet?
  4. Is there a mass in the neck, and is scintigraphy needed?
  5. When should the T4 be rechecked?
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Frequently asked questions

What are the warning signs of hyperthyroidism in dogs?

Drinking and urinating more, weight loss, restlessness, aggressiveness, panting and a fast heart rate. Some dogs show no signs and are found on routine blood work [4, 6].

What dog breeds are prone to thyroid issues?

For thyroid cancer, Golden Retrievers, Beagles and Siberian Huskies are over-represented, mostly between 10 and 15 years of age [1]. Golden and Labrador Retrievers were common among dogs with hormone-producing tumors [4]. For an underactive thyroid, see hypothyroidism in dogs.

What gets mistaken for hyperthyroidism?

A blood sample taken a few hours after a levothyroxine tablet, when T4 peaks [10]; thyroid hormone in raw food, gullets or treats [6, 7]; and, rarely, antibodies against T4 that interfere with the test, found in 0.5% of dogs in one study [11].

Can a raw diet cause hyperthyroidism in dogs?

Yes. Raw meat, gullets or head meat containing thyroid tissue can raise T4. Changing the food brought T4 back to normal in the dogs that were rechecked [6, 8].

How long do dogs with a functional thyroid tumor live?

In 70 dogs, median survival was 35.1 months overall and 72.6 months after surgery [5].

Sources

  1. Wucherer KL, Wilke V. Thyroid cancer in dogs: an update based on 638 cases (1995-2005). J Am Anim Hosp Assoc. 2010;46(4):249-254. doi.org/10.5326/0460249
  2. Enache D et al. Thyroidectomy in dogs with thyroid tumors: survival analysis in 144 cases. J Vet Intern Med. 2023;37(2):635-647. doi.org/10.1111/jvim.16644
  3. Broome MR, Peterson ME, Walker JR. Clinical features and treatment outcomes of 41 dogs with sublingual ectopic thyroid neoplasia. J Vet Intern Med. 2014;28(5):1560-1568. doi.org/10.1111/jvim.12406
  4. Frederick AN et al. Outcomes for dogs with functional thyroid tumors treated by surgical excision alone. J Am Vet Med Assoc. 2020;256(4):444-448. doi.org/10.2460/javma.256.4.444
  5. Scharf VF et al. Clinical features and outcome of functional thyroid tumours in 70 dogs. J Small Anim Pract. 2020;61(8):504-511. doi.org/10.1111/jsap.13183
  6. Köhler B, Stengel C, Neiger R. Dietary hyperthyroidism in dogs. J Small Anim Pract. 2012;53(3):182-184. doi.org/10.1111/j.1748-5827.2011.01189.x
  7. Broome MR et al. 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. 2015;246(1):105-111. doi.org/10.2460/javma.246.1.105
  8. Zeugswetter FK, Vogelsinger K, Handl S. Hyperthyroidism in dogs caused by consumption of thyroid-containing head meat. Schweiz Arch Tierheilkd. 2013;155(2):149-152. doi.org/10.1024/0036-7281/a000432
  9. Hare JE et al. Safety of orally administered, USP-compliant levothyroxine sodium tablets in dogs. J Vet Pharmacol Ther. 2018;41(2):254-265. doi.org/10.1111/jvp.12457
  10. van Dijl IC et al. Pharmacokinetics of total thyroxine after repeated oral administration of levothyroxine solution and its clinical efficacy in hypothyroid dogs. J Vet Intern Med. 2014;28(4):1229-1234. doi.org/10.1111/jvim.12363
  11. Piechotta M, Arndt M, Hoppen HO. Autoantibodies against thyroid hormones and their influence on thyroxine determination with chemiluminescence immunoassay in dogs. J Vet Sci. 2010;11(3):191-196. doi.org/10.4142/jvs.2010.11.3.191
  12. Morré WA et al. Thyrotoxicosis induced by excessive 3,5,3′-triiodothyronine in a dog. J Am Vet Med Assoc. 2017;250(12):1427-1431. doi.org/10.2460/javma.250.12.1427
  13. Carney HC et al. 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism. J Feline Med Surg. 2016;18(5):400-416. doi.org/10.1177/1098612X16643252
  14. Shadwick SR, Ridgway MD, Kubier A. Thyrotoxicosis in a dog induced by the consumption of feces from a levothyroxine-supplemented housemate. Can Vet J. 2013;54(10):987-989. pubmed.ncbi.nlm.nih.gov/24155422

Laboratory methods and reference intervals vary. This article explains; it does not diagnose. For a decision about your pet, use the original report and talk to your veterinarian. Our editorial policy.

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