The effects of hypothyroidism on strength-duration properties of peripheral nerve

Axonal injury and/or demyelination commonly result in peripheral mononeuropathy, polyneuropathy or entrapment neuropathies in hypothyroidism. In this study, we aimed to measure the strength-duration time constants (SDTC) of motor and sensory fibres in patients with primary hypothyroidism before and after hormone replacement treatment. The motor and sensory SDTC and rheobase of 14 patients with overt hypothyroidism (TSH>10 mg/ml, free T4<normal range, with positive thyroid autoantibodies) and 10 age-matched healthy volunteers were measured following stimulation of left ulnar nerve at the wrist. Nerve conduction velocity studies revealed carpal tunnel syndrome in 3 of 14 patients. When the values of patients and control group were compared, motor and sensory rheobase, thresholds using 0.1 ms and 1 ms stimulus for motor and sensory fibres were significantly different (p<0.05). The values were bigger in patient group reflecting a higher threshold. When the values of patients were compared before and after therapy, motor and sensory rheobase, thresholds using 0.1 ms and 1 ms stimulus for motor and sensory fibres were significantly different (p<0.05). When the values of 10 of 14 patients who had undergone hormone therapy were compared with the control group, there wasn't any significant difference. Some reports suggest that axonal injury develop in hypothyroidism. Our results suggest that even in the absence of polyneuropathy findings, an increased threshold and in consequence an increased axonal excitability which can improve after hormone therapy may be present in overt hypothyroidism. (C) 2010 Elsevier B.A. All rights reserved.