Etiology: The peripheral nerve is the most common target in the neurologic system for hypothyroidism. What causes a peripheral neuropathy due to hypothyroidism? I'm glad you asked....
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 is most common. This results in paresis without ataxia and reduced to absent peripheral spinal reflexes. Signs may be initially mild.
Cranial nerve deficits such as facial nerve paralysis (VII), vestibular dysfunction (VIII), or laryngeal paralysis (X).
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.
Cranial nerve deficits may persist even with appropriate treatment.
Frequency:
Common in older dogs! (And recently seen in one cat with vestibular signs. This job always keeps me on my toes, that's for sure!)