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 () (n = 17, mean age = 39 +/- 34 years), T4 = 9 +/- 0.7 micrograms/dL, TSH = 4.3 +/- 0.4 microU/mL, sTSH post-TRH (peak value) = 37.6 +/- 1.6 microU/mL; (ii) Hypothyroidism Stage II (group II) (n = 10, mean age: 43 +/- 6 years), T4 = 7.7 +/- 0.8 microgram/dL, TSH = 20 +/- 5 microU/mL, TSH post-TRH > 50 microU/mL; (iii) Control Group (n = 20, mean age 41 +/- 5 years), healthy subjects. All patients and controls were women. TRH test consisted in the i.v. injection of 200 micrograms TRH (normal peak value up to 25 microU/mL, normal basal TSH < 5.5 microU/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).(ABSTRACT TRUNCATED AT 250 WORDS)