On presentation to Cornell’s Neurology Service, the dog was quiet but responsive. His neurological exam revealed intermittent very subtle head tilt to the left, absent palpebral reflex and menace response bilaterally, and subtle positional nystagmus with fast phase to the right. The patient had normal symmetrical facial musculature, however, droopy lip commissures were appreciated The differential diagnoses for chronic and static left sided vestibular and bilateral facial nerve paresis, with mildly obtunded mentation, are metabolic disease, infectious/inflammatory disease, or multiple neoplasia. The plan was to do baseline blood work in addition to endocrine (thyroid and adrenal) testing, infectious disease testing, and if these were non-diagnostic, proceed to advanced imaging. Complete blood count and urinalysis were normal, however the serum biochemistry profile revealed hypercholesterolemia. The patient was seronegative for Toxoplasma gondii and Neospora caninum.

The bilateral CN VII deficit, tremor events and hypercholesterolemia were indicative of hypothyroidism, however, a unilateral CN VIII was unusual although possible with hypothyroidism where multiple cranial nerves may be affected. A thyroid panel was diagnostic for hypothyroidism with positive thyroglobulin autoantibodies. Thyroid hormone replacement therapy was initiated. The patient’s clinical signs improved per the owner 15 days after initiating therapy. Neurologic abnormalities are a rare presenting complaint in dogs with hypothyroidism that veterinarians should be aware of.

Axonal degeneration due to decreased metabolism in neurons, demyelination due to glycogen deposition, and hyperlipidemia have all been implicated in the etiology of neurologic abnormalities secondary to hypothyroidism.