Seizures and Hyperthyroidism in Cats: What's the Correlation?

My patient is a 12-year-old, female, DSH cat that first developed generalized seizures about 6 months ago. My workup at that time was fairly unremarkable, with normal results for a CBC, serum chemistry profile, and urinalysis.

Over the last few months, the seizures have continued (about 1 mild seizure a week). On recheck, the cat has lost about a pound in body weight and now has a thin body condition score (2/5) with moderate loss of muscle mass over her back. A small right thyroid nodule was palpated. No abnormalities were found on neurologic examination.

Routine blood tests were all within reference range limits but the serum concentrations of total T4 (5.1 μg/dl; reference range, 0.8-4.0 μg/dl) and free T4 (>100 pmol/L; reference range, 10-50 pmol/L) are both high.

So this cat is definitely hyperthyroid. My questions include the following:

• Can hyperthyroidism cause seizures? 
• If so, will treating the hyperthyroidism improve the seizure disorder? 
• How should I proceed with my workup and treatment?

My Response:

Hyperthyroidism is a rare cause of metabolic-induced seizures (1,2).  It is believed that high circulating thyroid hormone values may play a role in decreasing the seizure threshold in the brain by altering the level of neurotransmitters, as well as by directly increasing neuronal excitability. Hyperthyroidism also increases the oxygen and glucose demand to the brain, creating a potential for hypoxic and hypoglycemic-induced seizures.

In hyperthyroid cats, both focal or generalized seizures characteristic of epilepsy have been described, albeit rarely (2,3). These cats generally have chronic, severe hyperthyroidism, with very high serum T4 concentrations. After treatment of the hyperthyroidism, some cats will show marked improvement or even complete resolution of seizure activity (2).

Hyperthyroidism can also induce seizure activity via many other "indirect" mechanisms. For example, these cats may develop hypertension, which could result in a cerebral vascular accident (stroke) leading to seizures and other neurological deficits (3-7). Secondary cardiac disease is also common in hyperthyroid cats, so thromboembolism is always a remote possibility. Finally, some hyperthyroid cats with severe heart disease will develop syncopal episodes as a result of arrhythmias or heart block that can mimic seizure activity (7-9).

Bottom Line
Again, a hyperthyroid-induced seizure disorder is extremely rare in cats.  In almost all cats in which the seizure disorder resolves with successful treatment of the hyperthyroidism alone, the cats have very severe and advanced thyroid disease.  We must never forget that any late-onset seizure disorder is a cause for concern and that many other differentials must be considered.

In this cat, the hyperthyroidism is very mild and only recently diagnosed, making it less likely that the seizure disorder is directly related to the cat's thyroid disease. However, I would certainly recommend checking this cat's blood pressure, doing a good fundic exam, and evaluating thoracic radiographs and an ECG. If no secondary complications of hyperthyroidism are identified, I'd treat the cat's hyperthyroidism with methimazole to evaluate the neurological response to therapy.

If the seizures greatly improve or completely resolve, then it's likely that they are related to the hyperthyroidism. However, if the seizure activity continues or worsens in severity, other primary CNS causes for seizures (e.g.,  intracranial neoplasia, infection, or inflammation) is likely. In that case, a complete neurological evaluation, including CSF analysis and brain imaging, should be recommended.

References:

1. Aszalós Z. Some neurologic and psychiatric complications in endocrine disorders: the thyroid gland. Orvosi Hetilap 2007;18;148:303-310. 
2. Joseph RJ and Peterson ME (1992) Review and comparison of neuromuscular and central nervous system manifestations of hyperthyroidism in cats and humans. Progress in Veterinary Neurology 1992;3:114–119.
3. Mooney CT, Peterson ME. Feline hyperthyroidism. In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association; 2012:92-110.
4. Stepien RL. Feline systemic hypertension: Diagnosis and management. Journal of Feline Medicine and Surgery 2011;13:35-43. 
5. Jepson RE. Feline systemic hypertension: Classification and pathogenesis. Journal of Feline Medicine and Surgery 2011;13:25-34.
6. Morrow LD, Adams VJ, Elliott J, et al. Hypertension in hyperthyroid cats: prevalence, incidence and predictors of its development [abstract]. Journal of Veterinary Internal Medicine 2009;23:699.
7. Syme HM. Cardiovascular and renal manifestations of hyperthyroidism. Veterinary Clinics of North America: Small Animal Practice 207;37: 723–743.
8. Fox PR, Peterson ME, Broussard JD. Electrocardiographic and radiographic changes in cats with hyperthyroidism: comparison of populations evaluated during 1992–1993 vs 1979– 1982. Journal of the American Animal Hospital Association 1999;35:27–31.
9. Jacobs G, Hutson C, Dougherty J, et al. Congestive heart failure associated with hyperthyroidism in cats. Journal of the American Veterinary Medical Association 1986;188: 52–56.