Peripheral Neuropathy in Subclinical Hypothyroidism

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Abstract
Alterations in peripheral nerves are well documented in overt myxedema but not in subclinical hypothyroidism. We performed electrophysiologic studies to investigate such abnormalities in patients with normal serum total T4 and hyperresponsiveness of TSH to TRH, either with normal or high levels of basal circulating TSH. Subjects were divided in three groups: (i) Hypothyroidism Stage I (group I) (n = 17, mean age = 39 ± 4 years), T4 = 9 ± 0.7 μg/dL, TSH = 4.3 ± 0.4 μU/mL, TSH post-TRH (peak value) = 37.6 ± 1.6 μU/mL; (ii) Hypothyroidism Stage II (group II) (n = 10, mean age: 43 ± 6 years), T4 = 7.7 ± 0.8 μg/dL, TSH = 20 ± 5 μU/mL, TSH post-TRH > 50 μU/mL; (iii) Control Group (n = 20, mean age 41 ± 5 years), healthy subjects. All patients and controls were women. TRH test consisted in the iv injection of 200 μg TRH (normal peak value up to 25 μU/mL, normal basal TSH < 5.5 μ/mL). None of the patients had carpal tunnel syndrome or any other neurological or metabolic disturbances. We studied the distal motor latencies, motor and sensory amplitudes, and nerve conduction velocities. The motor parameters were measured in the median and external sciatic popliteal (ESP) nerves, and the sensory parameters in the median and sural nerves. In most cases values were obtained from both right and left nerves. Motor parameters: no differences were found between all groups for conduction velocities (CV). The motor distal latencies (MDL) in median nerves in groups I and II vs. controls were 3.6 ± 0.1 and 3.6 ± 0.2 vs 3.0 ± 0.1 msec (p < 0.05). The MDL in ESP nerves vs controls were 4.5 ± 0.1 and 4.9 ± 0.2 vs 4.2 ± 0.1 msec (p < 0.05). Amplitudes in group I were only decreased in the ESP nerves when compared to controls: 4.8 ± 0.8 vs 6.6 ± 0.5 mV (p < 0.05). However, group II showed a significant shortening of the amplitude in both the median and ESP nerves when compared to controls: 6.9 ± 0.9 vs 8.6 ± 0.2 mV (p < 0.05) and 3.4 ± 0.7 vs 6.6 ± 0.5 mV (p < 0.05), respectively. Sensory parameters: no differences were observed for CV between the three groups. The amplitudes were significantly decreased in sural nerves in groups I and II when compared to the control group: 26.8 ± 2.4 and 21.3 ± 2.0 vs 32.1 ± 2.5 μV (p < 0.05). However, measurements in median nerve showed significant differences only between group II and controls: 41.3 ± 3.8 vs 52.5 ± 4.1 μV (p < 0.05). As shown, MDL are increased in both stages of subclinical hypothyroidism; amplitudes are low in stage II but are affected in the leg nerves only in stage I. Nervous conduction speeds are normal in all groups. It is concluded that an incipient axonal alteration is present in subclinical hypothyroidism. The abnormality is more evident when basal levels of TSH are increased. However, most of these alterations are also present when hyperresponsiveness of TSH to TRH is the apparent unique abnormality of thyroid function.