The pattern-shift evoked potential was measured in 19 hypothyroid patients before treatment, and after short- (one week) and long-term (12 to 24 weeks) thyroid hormone replacement therapy. Before treatment, nine patients had an abnormally prolonged visual evoked potential latency, more than 115 msec. After one week of therapy with 50 μg per day of l-triiodothyronine, the mean visual evoked potential latency for the entire group was unchanged, 114 ± 8to114 ± 7 msec. However, long-term therapy with 100 to 200 μg per day of l-thyroxine significantly shortened the visual evoked potential latency to 105 ± 1 msec (p < 0.001). In eight of the nine patients with initially abnormal results, the visual evoked potential latency was completely restored to normal. The visual evoked potential amplitude was also significantly changed by long-term l-thyroxine, 4.8 ± 0.7 to 9.0 ± 1.0 μV (p < 0.01). It is concluded that the visual evoked potential is reversibly altered in hypothyroidism. This neurophysiologic parameter permits quantitation of the effects of hypothyroidism on the central nervous system and the extent and rate of response to thyroid hormone replacement therapy.
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