Background: Hypothyroidism is one of the greatest endocrinal disorders that can affect the central nervous system
Objective: To assess silent affection of central nervous system among cases of subclinical hypothyroidism.
Patients and methods: A group of Fifty cases of sub clinical hypothyroidismreferredfrom outpatients' clinic of Internal Medicine Department to Neurophysiology unit of Neurology Department of Al-Azhar University hospital in new Damietta, and  another euthyroid  group of fifty, age and gender matched healthy persons. The electrophysiological study of both groups was done including Electroencephalography and evoked potentials (auditory and visual).
Results: As regard to EEG, Eighteen patients (36%) had EEG abnormalities. thir teen patients (26%) had diffuse slowness of background activity formed mainly of theta wave activity with or without paroxysmal high voltage theta or delta wave activity (16%).Auditory evoked potentials (ABR)showed significant prolongation, of  III &V wave latencies ,  and inter peak latencies of
I-V, & III-V among the subclinical group on comparison with the control group, and  also of significant prolonged latency  of  wave  I of left ear  and without correlation with thyroid stimulating hormone serum level.  Visual evoked potentials (VEP) showed a significant prolongation of P100 latency in-group of sub clinical hypothyroidism on comparison with control group. There was no correlation between the TSH serum level and evoked potential latencies (ABR and VEP)
Conclusion: The central nervous system can be early affected in sub clinical hypothyroidism and the follow up electrophysiological assessment is recommended