Hypothyroidism is one of the most common endocrine diseases faced in small-animal medicine.1 While many practitioners are well-acquainted with the classic manifestations of canine hypothyroidism (weight gain, alopecia, lethargy, poor appetite), those involving the central nervous system, peripheral nervous system, and muscles are less well-recognized (Table 1).
Table 1: Clinical signs reflecting neurologic dysfunction in hypothyroid dogs
Although uncommon, it is important that practitioners be aware that dogs with hypothyroidism can present with neurologic deficits as the only clinical sign. Consequently, testing of the pituitary-thyroid axis may be indicated in certain clinical scenarios despite a lack of other clinical findings suggestive of hypothyroidism.
In this article, we use case examples to illustrate a variety of clinical scenarios in which neurologic signs and clinicopathologic test results should prompt the evaluation of the pituitary-thyroid axis and a possible diagnosis of hypothyroidism. Clinical evidence that prompted pituitary-thyroid axis testing in these patients included the diagnosis of syndromes that have been known to be associated with hypothyroidism such as facial nerve paralysis or vestibular disease, blood work abnormalities such as hypercholesterolemia, or the suspicion of an ischemic infarction. Although a causal relationship between hypothyroidism and neurologic deficits is difficult to establish, the response to therapy in these three cases helped support a diagnosis of neurologic dysfunction secondary to hypothyroidism.
Case 1: A recumbent Leonberger
A 4-year-old spayed female Leonberger was presented to the Matthew J. Ryan Veterinary Hospital at the University of Pennsylvania for evaluation of an acute onset of tetraparesis.
The owners reported that the dog had been normal until the previous morning, when it had experienced pelvic limb weakness, which progressed to recumbency over the course of several hours. Once recumbent, the dog had displayed an episode of rigidity involving all four limbs and diffuse muscle fasciculation. Before the onset of clinical signs, the dog had been healthy. The dog's vaccination status was up-to-date, and the dog was receiving heartworm and tick prevention. There was no history of exposure to toxins.
Physical and neurologic examinations
Physical examination abnormalities were limited to the nervous system. The dog's body condition score was ideal at 4/9.
On neurologic examination, the dog's mentation was normal. The dog was recumbent but able to walk when assisted. When prompted to walk, the dog displayed a short-strided gait in all four limbs and occasionally would knuckle onto the dorsum of the paw of the left thoracic limb. Postural reactions were abnormal in all four limbs. However, when the dog's weight was supported, the postural reactions were normal in all four limbs.
Patellar reflexes were decreased bilaterally and withdrawal reflexes were reduced in all four limbs. The muscular tone of the limbs was reduced, and no muscle atrophy was appreciated. The dog displayed a right-sided head tilt with normal physiologic nystagmus and no abnormal nystagmus. The menace response in the right eye was absent. Palpebral reflexes, pupil size, and pupillary light reflexes were normal. No other cranial nerve deficits were noted. The dog did not exhibit pain on palpation of the vertebral column or when moving the head and neck through normal range of movement.