Hypothyroidism means a dog's thyroid gland makes too little thyroid hormone, which slows the body's metabolism. It's one of the most common hormone disorders in dogs, but it's also one of the most overdiagnosed, because many illnesses and medicines lower thyroid test results.1,2 It can't be cured, but a daily tablet controls it well.3,1 This guide covers the signs, breeds at risk, how to get a reliable diagnosis, treatment, and what the evidence says about thyroid problems and behavior.
Talk to your vet if your dog:
- Gains weight without eating more, or seems unusually tired or cold-intolerant1,3
- Has thinning hair on both sides of the body or tail, or keeps getting skin or ear infections1,4
- Is on thyroid medication and becomes restless or hyperactive, sleeps poorly, pants, loses weight or drinks a lot, which can mean the dose is too high3,5
What the Research Shows
- About 1 in 435 dogs. Among 905,553 UK dogs, 0.23% had a hypothyroidism diagnosis.2
- Breed is a major factor. Dobermans had 17 times the odds of crossbreeds, followed by Tibetan Terriers, Boxers and Alaskan Malamutes; Pugs, Yorkies and Shih Tzus had lower odds.2
- It's a middle-age disease. The median age at first diagnosis was 7.7 years.2
- No single test is definitive. TSH is normal in 20% to 40% of truly hypothyroid dogs.1
- Treatment didn't change behavior long-term. In 20 hypothyroid dogs, activity rose at 6 weeks of treatment, but no behavior differed from baseline at 6 months.6
What Is Hypothyroidism?
More than 95% of cases are caused by damage to the thyroid gland itself, either by the immune system (lymphocytic thyroiditis) or by shrinkage of unknown cause.1,3 Thyroid hormone affects almost every organ, so low levels cause slow, wide-ranging changes.1
Signs of Hypothyroidism in Dogs
- Weight gain without a bigger appetite; over 40% of hypothyroid dogs are overweight1,3,4
- Lethargy, weakness or reluctance to exercise4,1
- Thinning hair or bald patches that usually aren't itchy, often on the sides and tail, in about two-thirds of dogs1
- Dry or dull coat, slow hair regrowth after clipping, and darkened skin1
- Repeated skin and ear infections1,4
- Feeling the cold or seeking warmth3,1
Blood tests often show high cholesterol, in about 80% of cases, and mild anemia.1 Because weight gain and tiredness are so common in middle-aged dogs anyway, these signs alone don't prove hypothyroidism.4
Which Dogs Get Hypothyroidism?
Most dogs are middle-aged; dogs aged 11 to 13 had 4.5 times the odds of dogs aged 5 to 7.2 Medium to large breeds are most often affected, and sex isn't a risk factor.1,2 High-risk breeds in the UK study were the Doberman Pinscher, Tibetan Terrier, Boxer and Alaskan Malamute.2
How Vets Diagnose Hypothyroidism
Merck calls hypothyroidism "probably one of the most overdiagnosed diseases in dogs," and stresses that a low total T4 alone isn't enough for a diagnosis.1
- Total T4 is a good screening test: a normal result makes hypothyroidism unlikely.1
- Free T4 by equilibrium dialysis is more specific, and some dogs with low total T4 turn out to be normal on this test.1,3
- TSH helps when it's high, but it's normal in 20% to 40% of hypothyroid dogs.1
- Avoid testing during other illness. Illness lowers thyroid results (euthyroid sick syndrome), as do many drugs, including steroids, phenobarbital, some antibiotics and some pain medicines.1
- Sighthounds such as Greyhounds and sled dogs normally have lower T4 levels.1
Treatment of Hypothyroidism
Treatment is a synthetic thyroid hormone, levothyroxine, given daily for life. FDA-approved versions for dogs include Thyro-Tabs Canine and ThyroKare (approved in 2021).5,7 In the field studies supporting approval, 76% and 81% of dogs respectively were treatment successes.5,7
Your vet will recheck blood levels, usually 4 to 8 weeks after starting, then once or twice a year.1 Energy often improves within about 2 weeks, but skin, coat and weight can take up to 3 months.1 When the diagnosis is right, treatment failure is uncommon.1
Does Hypothyroidism Cause Behavior Problems?
A link between low thyroid and aggression has been suggested for years, but the evidence is weak. In a study of 20 hypothyroid dogs, treatment increased activity at first, but no behavior had changed at 6 months.6 A UK thesis found low thyroid hormone was linked to lower, not higher, aggression.8 Dogs with behavior problems should have a full medical work-up, but thyroid tablets aren't a treatment for aggression in dogs with normal thyroid function.
Other Thyroid Problems in Dogs
An overactive thyroid is rare in dogs, unlike cats.1 It's most often caused by too high a dose of thyroid medicine,3 and occasionally by raw diets or treats containing thyroid tissue, or a hormone-producing thyroid tumor. A lump in the neck should always be checked by a vet. See thyroid cancer in dogs.
Frequently Asked Questions
What are the signs of hypothyroidism in dogs?
Weight gain, low energy, thinning hair on the body and tail, dull coat and repeated skin or ear infections.
What breeds are prone to hypothyroidism?
Dobermans, Tibetan Terriers, Boxers, Alaskan Malamutes and other medium to large breeds.
How do vets test for hypothyroidism in dogs?
Usually with total T4, followed by free T4 and TSH if needed. One low T4 result isn't enough to diagnose it.
Can hypothyroidism in dogs be cured?
No, but daily thyroid hormone tablets control it well for life.
How long until thyroid medication works in dogs?
Energy often improves in a few weeks; skin, coat and weight can take up to 3 months.
Can hypothyroidism make a dog aggressive?
The evidence for this is weak, and studies haven't shown thyroid treatment reliably changes behavior.
What happens if hypothyroidism isn't treated?
Signs such as weight gain, lethargy and skin problems usually get worse over time.
Is hypothyroidism common in dogs?
It affects about 1 in 435 dogs in UK practices, mostly middle-aged dogs.
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Sources and further reading
This article was rewritten and fact-checked on October 15, 2026 against current veterinary references and specialist guidelines:
- Heseltine J. Hypothyroidism in animals. Merck Veterinary Manual, May 2025.
- Geddes RF, O'Neill DG, Khoo JSP, et al. Frequency, breed predispositions and other demographic risk factors for diagnosis of hypothyroidism in dogs under primary veterinary care in the UK. Canine Medicine and Genetics, 2022 (VetCompass study).
- Weir M, Williams K, Ward E. Hypothyroidism in dogs. VCA Animal Hospitals, 2024.
- Daminet S. Canine hypothyroidism: clinical aspects, progress in diagnosis, pitfalls. EVC Voorjaarsdagen, 2008. Conference proceedings hosted by IVIS.
- LLOYD Inc. Thyro-Tabs Canine (levothyroxine sodium tablets) package insert, NADA 141-448.
- Hrovat A, De Keuster T, Kooistra HS, et al. Behavior in dogs with spontaneous hypothyroidism during treatment with levothyroxine. Journal of Veterinary Internal Medicine, 2019.
- U.S. Food and Drug Administration. FDA approves ThyroKare (levothyroxine sodium tablets) for replacement therapy for diminished thyroid function in dogs. CVM Update, January 2021.
- Barlow TA. Doctoral thesis on thyroid function and behaviour in dogs. University of Southampton, 2002.
About this article
Last updated : fully rewritten and combined with our former thyroid disease page, using a VetCompass study of 905,553 dogs, the Merck Veterinary Manual, FDA approval records, published behavior studies, VCA and IVIS-hosted proceedings; added the research summary, signs, breed risk, diagnosis and overdiagnosis, treatment, monitoring, behavior evidence, key takeaways and FAQ.
This page is for general information and is not a substitute for an exam by your veterinarian.
