Peripheral nerves resist damage from chronic hypothyroidism in dogs

Abstract

BACKGROUND: Hypothyroidism has been implicated in the development of multiple peripheral mono- and polyneuropathies in dogs. The objectives of this study were to evaluate the clinical and electrophysiologic effects of experimentally induced hypothyroidism on the peripheral nervous system of dogs. HYPOTHESIS: Chronic hypothyroidism will induce peripheral nerve sensorimotor dysfunction. ANIMALS: Eighteen purpose-bred, female dogs. METHODS: Prospective, longitudinal study: Hypothyroidism was induced by radioactive iodine administration in 9 dogs, and the remaining 9 served as untreated controls. Neurological examinations were performed monthly. Electrophysiologic testing consisting of electromyography (EMG); motor nerve conduction studies of the sciatic-tibial, radial, ulnar, and recurrent laryngeal nerves; sciatic-tibial and ulnar F-wave studies; sensory nerve conduction studies of the tibial, ulnar, and radial nerves; and evaluation of blink reflex and facial responses were performed before and 6, 12, and 18 months after induction of hypothyroidism and compared with controls. RESULTS: Clinical evidence of peripheral nervous dysfunction did not occur in any dog. At 6 month and subsequent evaluations, all hypothyroid dogs had EMG and histologic evidence of hypothyroid myopathy. Hypothyroid dogs had significant (P<or=.04) decreases in ulnar and sciatic-tibial compound muscle action potentials over time, which were attributed to the concurrent myopathy. No significant differences between control and hypothyroid dogs were detected in electrophysiologic tests of motor (P>or=.1) or sensory nerve conduction velocity (P>or=.24) or nerve roots (P>or=.16) throughout the study period, with values remaining within reference ranges in all dogs. CONCLUSION: Chronic hypothyroidism induced by thyroid irradiation does not result in clinical or electrophysiologic evidence of peripheral neuropathy, but does cause subclinical myopathy.