Hypothyroidism

28 Hypothyroidism



David L. Panciera



1. What causes hypothyroidism in dogs?

Autoimmune thyroiditis is responsible for destruction of the thyroid gland in approximately 50% of cases. These dogs usually have circulating antithyroglobulin antibodies and occasionally have antibodies against thyroid hormones. Most other cases of hypothyroidism are the result of idiopathic thyroid gland atrophy. The cause is unknown, but long-standing autoimmune thyroiditis has been suspected. Rarely, hypothyroidism results from pituitary disease such as a tumor.


2. What abnormalities are most often evident from the history or physical examination of hypothyroid dogs?

Dermatologic abnormalities, including alopecia, poor hair coat, seborrhea, or pyoderma, are found in about 90% of hypothyroid dogs. Alopecia may initially occur in areas of friction, such as the tail and neck, and eventually spread to involve bilaterally symmetrical areas on the trunk. Obesity and lethargy are common. Lethargy is often reported by owners only in retrospect, when they notice how activity level increases after treatment. Bradycardia or a low-normal heart rate is also common.


3. What neurologic abnormalities are caused by hypothyroidism?

Hypothyroidism can cause localized or generalized peripheral neuropathies as well as abnormalities of the central nervous system. Generalized weakness, hyporeflexia, and decreased conscious proprioception occur with generalized neuropathy, although these signs are typical of peripheral neuropathy due to any cause. Localized neuropathies result in facial nerve and/or vestibular nerve dysfunction. Hypothyroidism-associated neuropathy has also been reported, although rarely, to cause unilateral forelimb lameness. Other clinical signs of hypothyroidism may or may not be present in dogs with neuropathy. Central nervous system abnormalities such as ataxia, hemiparesis, nystagmus, hypermetria, circling, and cranial nerve deficits can also be due to hypothyroidism.


4. Does hypothyroidism have important effects on cardiovascular function?

Myocardial function is impaired by hypothyroidism, but clinical signs are rare. Bradycardia, low-voltage complexes on the electrocardiogram, and decreased myocardial contractility are relatively frequently seen. Rarely, clinical signs of heart failure due to hypothyroidism-induced dilated cardiomyopathy may be seen.


5. Does hypothyroidism cause megaesophagus and laryngeal paralysis?

Megaesophagus and laryngeal paralysis have not been proven to be caused by hypothyroidism. While these problems may occur in hypothyroid dogs, thyroid hormone supplementation rarely if ever resolves the problem.


6. When evaluating a dog for aggression, should hypothyroidism be considered a possible cause?

It appears that a small proportion of dogs with aggression problems have hypothyroidism as an underlying cause. The dogs in which aggression was attributed to hypothyroidism had dominance aggression and few other signs of hypothyroidism. A complete response may not occur with thyroid hormone supplementation alone, and it is recommended that appropriate behavioral modification treatment be instituted concurrently.


7. Is reproductive function affected by hypothyroidism?

Hypothyroidism appears to have little effect on reproduction in the male dog. Little objective information is available regarding the effects of hypothyroidism on female reproduction, although prolonged interestrous intervals, failure to cycle, silent estrous cycles, lack of libido, and prolonged estrual bleeding have been reported to be caused by hypothyroidism.

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Jul 31, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Hypothyroidism

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