From NeuroPetVet, written by Dr. Mark Troxel, one of the big names in veterinary neurology...
HypothyroidismHypothyroidism is one of the most common endocrine disorders encountered in dogs. The reported incidence ranges from 0.2 to 0.8% of the canine population. Clinical hypothyroidism results from low triiodothyronine (T3) and thyroxine (T4), the active thyroid hormones produced by the thyroid glands. Primary hypothyroidism accounts for 95% of cases and is most commonly due to lympocytic thyroiditis or idiopathic thyroid atrophy. There are many other reported causes, but the incidence is very low. For an in-depth review of this condition, please consult an endocrine textbook or see the articles listed at the bottom of the page. SignalmentHypothyroidism is most common in middle-aged to older dogs. Approximately 1/3 of patients have onset of clinical signs between 4 and 6 years of age, while approximately 22% of cases each occur from 2-4 years of age or 7-9 years of age. Congenital hypothyroidism is rare in dogs. Clinical hypothyroidism is also uncommon to rare in cats. Clinical signsHypothyroidism causes a wide range of clinical signs affecting multiple body systems. Common clinical signs include weight gain, lethargy, mental dullness, skin disorders (hyperkeratosis, symmetric alopecia, thin hair coat, seborrhea, hyperpigmentation, etc.), gastrointestinal disease (e.g., esophageal hypomotilitiy), cardiovascular dysfunction (e.g., bradycardia), neuromuscular disease, reproductive problems and many others. A cause-and-effect relationship has not been established for most of the disorders listed below, and some of the conditions that have been “linked” to hypothyroidism are controversial.
PERIPHERAL NERVOUS SYSTEM DYSFUNCTION ASSOCIATED WITH HYPOTHYROIDISM
Cranial nerve dysfunction
Hypothyroidism has been associated with peripheral cranial nerve dysfunction of several cranial nerves, most commonly the facial nerve, vestibulocochlear nerve, and sensory branch of the trigeminal nerve. Facial nerve paralysis has been reported in up to 70% of dogs with clinical hypothyroidism and nerve dysfunction. Clinical signs of facial nerve paresis / paralysis include drooping ear, facial droop, inability to blink, and decreased tear production.
Peripheral vestibular dysfunction
Peripheral vestibular dysfunction has been linked to hypothyroidism. Clinical signs include head tilt, horizontal or rotary nystagmus, and vestibular ataxia. The vestibular signs may resolve in time, but recovery may take several weeks after beginning thyroid hormone supplementation.
Hypothyroidism can be a cause of laryngeal paralysis. In one study, 30 of 140 dogs with laryngeal paralysis had concurrent hypothyroidism. There is little information in the veterinary literature regarding treatment and recovery from laryngeal paralysis in hypothyroid dogs. Most consider laryngeal paralysis to be a surgical disease, but hypothyroidism should be treated nonetheless.
Hypothyroidism is commonly listed as a possible cause of megaesophagus in dogs. However, recent studies suggest this may not necessarily be true.
Concurrent myasthenia gravis and hypothyroidism has been reported in dogs. In human medicine, approximately 10-20% of myasthenic patients have concurrent thyroid disease. Some speculate that there is cross-reactivity between anti-acetylcholine receptor antibodies and thyroid antigens.
Hypothyroidism has been associated with a generalized polyneuropathy in dogs, most commonly in large and giant breed dogs. Clinical signs are typical of a generalized neuropathy, including generalized weakness, postural reaction deficits, hyporeflexia, decreased muscle tone, and muscle atrophy. Electrodiagnostics and nerve/muscle biopsy are occasionally performed, but laboratory testing often negates the need for these tests. Clinical signs often improve quickly with thyroxine supplementation and many dogs recover fully within 1-2 months.
Hypothyroid dogs may show signs of a generalized myopathy, including generalized weakness, stiffness, and muscle pain. The incidence in dogs is uncertain, but up to 40% of humans with hypothyroidism has skeletal muscle weakness.
CENTRAL NERVOUS SYSTEM DYSFUNCTION ASSOCIATED WITH HYPOTHYROIDISM
Central vestibular dysfunction
Hypothyroidism is most commonly associated with peripheral vestibular dysfunction, but it has also been reported in dogs with central vestibular dysfunction. There are only a handful of cases described in the veterinary literature, with several of the dogs having an infarction
Seizures and other forebrain signs
It is unclear if there is truly a direct cause-and-effect relationship for hypothyroidism as a cause of seizures. It’s been hypothesized that hyperlipidemia and atherosclerosis that occurs with hypothyroidism may be the cause of some patients’ seizures, as well as other forebrain signs, such as mental dullness, behavior change, and compulsive circling. It is also possible that hypothyroidism exacerbates seizure activity, making it more difficult to control seizures, rather than being a direct cause of seizures. In one retrospective study of 96 dogs with metabolic or toxic causes of seizures, only 3% of the dogs were hypothyroid. This finding, however, is limited by the retrospective nature of the study and the fact that there was little information provided about the hypothyroid dogs.
Myxedema coma in a Labrador Retriever with severe hypothyroidism. Note the edematous lips and eyelids. The dog was stuporous upon presentation to the hospital. He improved quickly after being given IV levothyroxine and returned to normal over the following 2 weeks with oral levothyroxine supplementation.
This is a rare manifestation of severe hypothyroidism in dogs that may be the result of abnormal neurotransmitter release/uptake or failure of thyroid hormone to enter the brain. Common clinical signs are thickened skin due to accumulation of mucopolysaccharides and hyaluronic acid, which bind water leading to pitting edema of the face, bradycardia, and stupor to coma. DiagnosisThe “classic picture” of hypothyroidism is low serum total T4 and free T4 levels and elevated TSH levels, especially when combined with compatible clinical signs. Nonregenerative anemia and hypercholesterolemia help further support the diagnosis.
Screening total T4 is commonly performed, often as part of a “package” including a CBC & biochemical profile. Normal total T4 means that the patient is almost certainly euthyroid. However, low total T4 may be due to hypothyroidism or euthyroid sick syndrome, a condition characterized by low total T4 and/or free T4 secondary to non-thyroidal illness or stress. Free T4 is the most accurate test of thyroid function if only one test can be performed. If the TSH level is high in patient with low free T4, hypothyroidism is likely. However, TSH alone cannot be used as an indicator of hypothyroidism as 20% of hypothyroid dogs have a low free T4, but a normal TSH level. TreatmentHypothyroidism is treated by supplementation with levothyroxine. The starting dose is 10-20 µg/kg (0.01-0.02 mg/kg) PO q12hr. The T4 level should be measured 4 weeks later 4-6 hours post-pill. Myxedema coma should be treated intravenously (5 µg/kg IV q12hr) since levothyroxine may not be well-absorbed orally in these patients. PrognosisThe prognosis for recovery is generally good, but it may take several weeks to months to recovery fully.