Low thyroid function (hypothyroidism) can cause seizures in dogs, though it’s one of the less common neurological signs of the condition. In a study of 21 hypothyroid dogs with neuromuscular problems, about 10% experienced seizures. More frequently, hypothyroidism causes other neurological issues like weakness in the limbs, facial nerve problems, and balance disturbances. Still, the connection between thyroid function and seizure activity is real, and understanding it matters because thyroid-related seizures can often be controlled by treating the underlying thyroid problem.
How Low Thyroid Affects the Brain and Nerves
Thyroid hormones regulate metabolism throughout the body, including in the brain and nervous system. When those hormone levels drop too low, nerve function can deteriorate in several ways. The most common neurological signs in hypothyroid dogs are head tilting, loss of coordination (ataxia), circling, and abnormal eye positioning. A study of 29 dogs with neurological signs from hypothyroidism found that lower motor neuron problems, vestibular (balance) deficits, megaesophagus, and laryngeal paralysis were the most frequent issues, in that order.
Seizures fit into this picture as a less typical but documented consequence. Thyroid hormones influence brain structure and function directly. Neuroimaging studies in both laboratory animals and humans confirm that the brain is physically altered in hypothyroid states. In dogs, this can lower the seizure threshold, meaning the brain becomes more susceptible to abnormal electrical activity that triggers a seizure. Dogs with severe, prolonged hypothyroidism are at higher risk for these kinds of central nervous system effects.
What Hypothyroid Seizures Look Like
Seizures from hypothyroidism don’t look different from seizures caused by other conditions. Your dog may collapse, paddle their legs, lose consciousness, drool excessively, or appear confused and disoriented afterward. The key clue that hypothyroidism might be the underlying cause is the presence of other symptoms that point to low thyroid function: unexplained weight gain, chronic skin problems, hair loss (especially on the trunk and tail), lethargy, and cold intolerance. A dog that develops seizures alongside these classic signs deserves a thyroid workup.
Beyond seizures, watch for subtler neurological changes. Facial asymmetry (one side of the face drooping), difficulty swallowing or regurgitating food, wobbliness when walking, and a head tilt can all signal that hypothyroidism is affecting the nervous system. In the study of 21 affected dogs, weakness in all four limbs was actually the most common neurological sign at nearly 29%, followed by facial asymmetry and regurgitation from megaesophagus at about 24% each.
The Diagnostic Challenge
Diagnosing hypothyroidism as the cause of seizures isn’t always straightforward. Your vet will run a thyroid panel measuring total T4, free T4, and TSH levels. But “normal” ranges vary significantly between breeds. English Setters, for example, naturally run T4 levels around 1.53 µg/dL, while Keeshonds average 2.25 µg/dL. Many purebred dogs have T4 and free T4 concentrations that sit at or below the lower limits of standard reference ranges, even when their thyroid is functioning normally. This means a result that looks low for one breed might be perfectly healthy for another.
To complicate things further, if your dog is already on anti-seizure medication, the test results can be misleading. Phenobarbital, one of the most commonly prescribed seizure medications for dogs, directly interferes with thyroid hormone levels. In a controlled study of 12 dogs given phenobarbital over 29 weeks, both total T4 and free T4 dropped significantly, while cholesterol rose. TSH (the hormone that signals the thyroid to produce more) showed only a delayed, incomplete compensatory increase. This means a dog on phenobarbital can appear hypothyroid on blood work even when their thyroid gland is perfectly healthy. Your vet needs to account for any current medications when interpreting thyroid results.
Treatment and Recovery Timeline
The encouraging news is that when hypothyroidism is genuinely behind the seizures, treating the thyroid problem often resolves or significantly reduces them. Dogs are placed on daily thyroid hormone replacement, and the general guideline is to allow six to eight weeks to see improvement in clinical signs. Lethargy and mental dullness tend to improve first, sometimes within the first couple of weeks. Neurological signs, including seizures, can show improvement rapidly once hormone levels normalize, though skin and coat changes take longer.
If seizures are frequent or severe, your vet may prescribe anti-seizure medication alongside thyroid treatment to keep your dog safe during the recovery window. Once thyroid levels stabilize and seizure activity stops, the anti-seizure medication can sometimes be tapered, though this decision depends on your dog’s individual response. Dogs that have been hypothyroid for a long time before diagnosis may have more persistent neurological effects than those caught early.
Other Causes to Rule Out
Because seizures from hypothyroidism are relatively uncommon, your vet will likely investigate other possibilities at the same time. Idiopathic epilepsy (seizures with no identifiable cause) is the most common reason for seizures in dogs between one and five years old. Other potential causes include liver disease, toxin exposure, brain tumors, infections, and low blood sugar. Hypothyroidism tends to appear in middle-aged dogs, typically between four and ten years old, so a dog in that age range with new-onset seizures plus classic thyroid symptoms is a strong candidate for testing.
The overlap between conditions matters practically. A dog could have both epilepsy and hypothyroidism. Or a dog being treated for epilepsy with phenobarbital could develop artificially low thyroid readings, leading to an unnecessary hypothyroidism diagnosis. Getting the sequence right, understanding which came first and whether the blood work reflects a true thyroid problem or a medication side effect, is what leads to the right treatment plan.