Does hypothyroidism affect the nervous system? You bet! The peripheral nerve is the most common target in the neurologic system. How does a low thyroid hormone affect the peripheral nerve (you might ask)? I'm glad you asked...
Etiology:
1) accumulation of mucinous deposits resulting in nerve entrapment
2) demyelination secondary to Schwann cell defect
3) vascular nerve damage secondary to hypothyroid induced dysfunction of BBB
4) disruption of axonal transport
Signalment:Typically older dogs, however congenital disease does (rarely) happen. NOTE: dogs are not always overweight, heat-seeking or have flaky-hair coats with peripheral neuropathy signs.
Clinical signs:A polyneuropathy (multiple nerves affected) is most common. This results in paresis without ataxia and reduced to absent peripheral spinal reflexes. Signs may be mild, to start.
Cranial nerve deficits such as facial nerve paralysis (VII), vestibular dysfunction (VIII), or laryngeal paralysis (X) are common signs of hypothyroidism. Of course, other things can cause damage to these nerves but don't forget to include hypothyroiism on your differential diagnoses list for patients with a peripheral neuropathy. Hypothyroidism may also cause a myopathy and/or megaesophagus.
Diagnostic tests:T4 is a good first step. If abnormal, a full panel is recommended.
Treatment options:You guessed it...supplementation!
Prognosis:The neuropathy is likely to improve a little or a lot, after several months with therapy if the axonal degeneration is not too severe. I commonly caution owners to be prepeared for perminent deficits and rejoice when that doesn't happen!
Cranial nerve deficits may persist even with appropriate treatment.
Frequency:
Common in older dogs. Remember they don't need to be over0-weight, heat seaking or even sluggish to get a hypothyroid induced peripheral neuropathy. Cats can be hypothyroid too!
May the luck of the Irish be with you this week! Thanks for reading and have a great week!