Hypothyroidism in Dogs and Seizures: Is There a Link?

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-10-02

Hypothyroidism in dogs and seizures are two separate conditions in most cases, and thyroid disease is not a common cause of true epilepsy. That said, low thyroid hormone can affect the brain and nervous system, and it can also cause episodes that look like seizures but are not. If your dog has both problems, your veterinarian needs to sort out which is which before treatment can work.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Answer

• Hypothyroidism is a hormone deficiency that slows your dog's metabolism. It is most common in dogs 4 to 10 years old and in mid to large breeds [1].
• True epilepsy and hypothyroidism are usually diagnosed as separate conditions. Hypothyroidism is not a well established cause of classic epilepsy in dogs.
• Low thyroid hormone can cause weakness, wobbliness, or fainting that owners may mistake for a seizure.
• Diagnosis requires blood tests, and a single low T4 result is not enough on its own to confirm hypothyroidism.
• Treatment is lifelong daily thyroid hormone replacement, and most dogs improve within weeks to a few months [2].

What Hypothyroidism Means in Plain Words

Hypothyroidism is the condition that results from thyroid hormone deficiency [2]. The thyroid gland sits in your dog's neck and makes hormones that control how fast the body uses energy. When those hormones drop, almost every cell in the body slows down [3].

More than 95% of hypothyroidism cases in dogs come from destruction of the thyroid gland itself, a form called primary hypothyroidism [2]. The two most common causes in adult dogs are lymphocytic thyroiditis and idiopathic atrophy of the thyroid gland [2]. A smaller number of cases come from a pituitary gland problem, and rarely a dog is born with a thyroid gland that does not develop or work normally [1].

This matters for the seizure question because thyroid hormones affect the nervous system along with everything else. When they are low, nerve and muscle function can change.

Signs Owners Notice First

Hypothyroidism develops slowly, so early signs are easy to miss [3]. The classic picture includes lethargy, weight gain or obesity, and coat changes [2]. You might see a dog who sleeps more, gains weight on the same food, and develops a dull, thinning coat.

Other signs can show up in many organ systems because thyroid hormones touch nearly every cell [3]. Some dogs get repeated ear infections with red, swollen, and smelly ears [3]. Skin problems, hair loss, and cold intolerance are common too.

Seizure-like signs are different. A true seizure typically involves loss of consciousness, stiffening, paddling, and involuntary urination or drooling. A study of owner-reported signs in epileptic dogs found that autonomic signs were extremely common, with ptyalism (drooling) reported in 87.6% of dogs, urination in 79.1%, and dilated pupils in 71.8% [4]. Those signs point toward a seizure instead of a thyroid problem.

If your dog has episodes that you cannot clearly describe, video them. Owners correctly classified paroxysmal events in 90.1% of cases in one survey, and video footage supported the seizure type in 96.5% of submitted clips [5]. A short phone video is one of the most useful things you can bring to your appointment.

Can Hypothyroidism in Dogs Cause Seizures?

This is the question most owners are really asking, and the honest answer is that the link is weak and not well established in veterinary research. Hypothyroidism is not recognized as a common cause of epilepsy in dogs. Idiopathic epilepsy, meaning epilepsy without an identified cause, is the most common chronic nervous system disorder of dogs, and its cause is poorly understood [6].

There are a few ways the two conditions can overlap in real life.

First, low thyroid hormone can cause generalized weakness, exercise intolerance, and a slow, stiff gait. A dog who suddenly goes weak and collapses may look like they had a seizure to an untrained eye.

Second, some dogs with hypothyroidism also have other hormone or immune conditions. Hypothyroidism occurs as a rare concurrent condition in dogs with hypoadrenocorticism, sometimes called Addison's disease, and it can appear at any phase of treatment [7]. When a dog has more than one endocrine problem, signs can blend together and be hard to untangle.

Third, some conditions that cause seizures can also affect thyroid test results. Dogs with chronic diarrhea and low albumin had significantly lower thyroid hormone levels and higher thyroid stimulating hormone levels, and 12.9% of those dogs had elevated TSH [8]. In other words, a sick dog can have abnormal thyroid numbers without actually having hypothyroidism. This is why a diagnosis cannot rest on one blood value.

If your dog has confirmed hypothyroidism and also has seizures, the two are usually treated as separate problems. Treating the thyroid alone is not expected to stop true epilepsy.

Causes and Risk Factors

Hypothyroidism is most common in dogs 4 to 10 years old [1]. It usually affects mid to large size breeds and is rare in toy and miniature breeds [1]. Breeds most commonly affected include the Golden Retriever, Doberman Pinscher, Irish Setter, Miniature Schnauzer, Dachshund, Cocker Spaniel, and Airedale Terrier [1].

It occurs equally in males and females, though spayed females may have a higher risk than unspayed dogs [1]. Medium to large breeds are more at risk, as are dogs that have had cancer [3].

Diet can matter in rare cases. One report described a dog that developed primary hypothyroidism with an enlarged thyroid gland from dietary iodine deficiency while eating a non-standardized home-prepared diet without vitamin and mineral supplementation [9]. Commercial canine diets typically meet iodine requirements, so this is uncommon [9].

For seizures, the risk factors are different. Genetics, breed, age at first seizure, and brain disease all play a role, and the cause of idiopathic epilepsy remains poorly understood [6].

How Veterinarians Tell Them Apart

Your veterinarian starts with a full physical examination and a detailed history [3]. Bring notes on when episodes happen, how long they last, what your dog does before, during, and after, and whether your dog is conscious.

Blood work is the next step. Diagnosis of hypothyroidism is based on clinical signs plus low serum thyroid hormone concentrations [2]. A typical panel measures total thyroxine (T4), free thyroxine (fT4), and thyroid stimulating hormone (TSH) [10]. Dogs with hypothyroidism have decreased T4 and fT4 and increased TSH [10].

Your vet may also run urine and cholesterol tests as part of the workup [3]. If the picture is unclear, repeat testing after a few weeks can help, because illness and some medications can push thyroid values down temporarily.

For the seizure side, your vet will want to know the semiology, meaning what the episodes actually look like [5]. Brain imaging and electroencephalography may be recommended in some cases [5]. If your dog has both low thyroid values and seizures, your vet may treat the thyroid first and watch whether the episodes change. If they do not, a separate seizure workup is needed.

You can read more about the testing process in Canine Hypothyroidism: Diagnosis and Management.

Treatment Options

Treatment for hypothyroidism is lifelong, daily oral thyroid hormone replacement [2]. The medication is levothyroxine sodium. Your veterinarian calculates the dose from your dog's weight and health, then adjusts it based on follow-up blood tests. There is no single dose that fits every dog.

Monitoring matters. In one study, dogs with hypothyroidism had decreased T4 and fT4 and increased TSH, along with signs of oxidative stress, and those markers changed after 45 days of levothyroxine treatment [10]. That is why recheck testing is scheduled instead of optional.

If your dog also has seizures, seizure medication is handled separately. In epileptic dogs, phenobarbital administration had a significant effect on seizure control [6]. Your veterinarian will consider how any seizure drug interacts with thyroid treatment and how it affects testing.

For a closer look at what treatment involves day to day, see Hypothyroidism in Dogs: Symptoms, Diagnosis & Treatment and Hypothyroidism in Dogs: Weight Gain, Lethargy and Thyroxine Dosing.

Recovery, Outlook, and Long-Term Management

Most dogs respond well to thyroid replacement. Energy often improves first, and coat and skin changes take longer because hair grows slowly. Your veterinarian will set a recheck schedule and adjust the dose based on blood work, not on how your dog looks alone.

If your dog has both conditions, expect two separate treatment plans and two separate monitoring schedules. Keep a seizure log with dates, times, duration, and a video when possible. This record helps your vet judge whether seizure control is improving.

Do not stop or change thyroid medication on your own. Even if your dog seems completely normal, the medication is what keeps them normal. If you miss a dose, call your veterinarian for guidance instead of doubling up.

Dogs with hypoadrenocorticism need extra attention because hypothyroidism can develop at any phase of treatment for that condition, and monitoring TSH levels may help catch early changes [7].

Prevention

There is no proven way to prevent hypothyroidism in dogs. The exact cause is unknown, and there are no known genetic causes [3]. What you can do is manage risk and catch problems early.

Feed a complete and balanced commercial diet, or work with your veterinarian if you home prepare food, since iodine deficiency has been reported in a dog on an unsupplemented home-prepared diet [9]. Keep routine wellness visits so your vet can track weight, coat, and energy changes over time. If your dog is in a higher-risk breed or age group, mention any slowing down at your next visit instead of waiting.

For seizure prevention, there is no known diet or supplement that prevents idiopathic epilepsy. The gut microbiome differs modestly in dogs with idiopathic epilepsy, but the cause of the disease remains poorly understood, so no prevention strategy is established [6].

Limitations and When to Contact a Veterinarian

The link between hypothyroidism and seizures is not well established, so no article can tell you what is happening in your individual dog. Your dog needs a veterinarian who can examine them, review video of the episodes, and run the right tests.

Call your veterinarian the same day if your dog has a first seizure, if episodes are getting more frequent or longer, if your dog seems confused or wobbly after an episode, or if you notice new lethargy, weight gain, or coat changes.

Go to an emergency clinic now if a seizure lasts more than a few minutes, if your dog has more than one seizure in a short period, if your dog does not fully wake up between episodes, or if your dog has trouble breathing or turns blue. Cyanosis was reported in only 2% of epileptic dogs in one survey, so blue gums are a serious sign [4].

Schedule a routine visit if you suspect thyroid disease based on slow changes like weight gain, thinning coat, or low energy, or if you want to discuss testing for a dog in a high-risk breed.

Frequently Asked Questions

Can hypothyroidism cause seizures in dogs?

It is not recognized as a common cause of true seizures. Low thyroid hormone can cause weakness or collapse that looks like a seizure, and some dogs have both conditions at the same time for unrelated reasons.

What do seizures in dogs look like?

Most true seizures involve loss of consciousness, stiffening, paddling, drooling, and urination. Drooling was reported in most epileptic dogs in one owner survey, along with urination and dilated pupils [4].

How is hypothyroidism diagnosed in dogs?

Diagnosis uses your dog's signs plus blood testing for T4, free T4, and TSH. Dogs with hypothyroidism typically have low T4 and free T4 with high TSH [10]. A single abnormal value is not enough, so your vet may repeat testing.

Is thyroid medication for dogs lifelong?

Yes, treatment is lifelong daily oral thyroid hormone replacement [2]. Your veterinarian sets the dose based on your dog's weight and health and adjusts it with follow-up blood work.

Can treating hypothyroidism stop my dog's seizures?

Usually not, if the seizures are true epilepsy. Treating the thyroid is expected to fix the metabolic problems, while seizures need their own evaluation and treatment plan. Talk with your veterinarian about both conditions together.

Related Articles

• Canine Hypothyroidism: Diagnosis and Management
• Hypothyroidism in Dogs: Symptoms, Diagnosis & Treatment
• Hypothyroidism in Dogs: Weight Gain, Lethargy and Thyroxine Dosing
• Levothyroxine Dog Dose: Hypothyroidism Dose and TSH Monitoring
• What Are the Side Effects of Thyroid Medication for Dogs?
• Addisons Disease in Dogs: Hypoadrenocorticism Crisis Electrolyte Balance

References

1. Disorders of the Thyroid Gland in Dogs - Dog Owners. Merck Veterinary Manual.
2. Hypothyroidism in Animals - Endocrine System. Merck Veterinary Manual.
3. Hypothyroidism in Dogs and Thyroid. Pet Poison Helpline.
4. Owner reported prevalence of autonomic signs in epileptic dogs. Frontiers in veterinary science, 2026.
5. Development of a Novel Epilepsy and Dyskinesia Survey for Large-Scale Characterization of Seizure Semiology in Dogs. Journal of veterinary internal medicine, 2025.
6. Gut microbiome alterations in canine idiopathic epilepsy: a pairwise case-control study. Animal microbiome, 2026.
7. Longitudinal assessment of thyroid function in dogs with hypoadrenocorticism: Clinical outcomes and prevalence of autoantibodies. Journal of veterinary internal medicine, 2025.
8. Hypoalbuminemia and altered thyroid parameters in dogs with chronic diarrhoea: Is protein loss driving TSH elevation?. Veterinary journal (London, England : 1997), 2026.
9. Case Report: Diet-associated iodine-deficiency goitrous hypothyroidism in a dog: a longitudinal clinical, endocrine, and ultrasonographic evaluation. Frontiers in veterinary science, 2026.
10. Redox status of biomarkers in serum of dogs with hypothyroidism and its treatment with levothyroxine sodium. Frontiers in veterinary science, 2025.

Further Reading

• Merck Veterinary Manual: Dog Owners
• Cornell Richard P. Riney Canine Health Center: Canine Health Information
• American Veterinary Medical Association: Pet Care