Changes of event related potential and cognitive processes in patients with subclinical hypothyroidism after thyroxine treatment.

OBJECTIVE: Recently, it has been repeatedly shown that patients with subclinical hypothyroidism (SH) formerly considered as completely symptom free, may have numerous minimal, often non-specific subjective complaints, and that in those patients it is possible to prove many subtle but objective deviations. We decided to quantify whether there are event related potential (ERP) deviations as electrophysiological markers of cognitive activity in patients with SH and whether ERP could be influenced by thyroxine treatment leading to normalization of TSH level in serum.

SUBJECTS AND DESIGN: Event related potential (ERP) was examined in thirty one patients (mean age 52 +/- 12.5 years) with SH and without any other endocrine or metabolic diseases and in 29 subjects of the control group. From 31 patients 20 women (mean age 61.8 +/-6.8 years) were selected and divided into a group of 10 women treated six months with L-thyroxine until the normalization of TSH and remaining 10 women receiving placebo. ERP examination was repeated and all such patients also underwent neuropsychological examination consisting of the Wechsler Memory Scale and the MMPI/100 (Minnesota Multiphasic Personality Inventory). The interval between the diagnosis of SH and final evaluation of treatment was 16 months.

RESULTS: In SH thyroxine treated patients the average P3 wave latency was 374 ms (SD 40.6), while in placebo group it was 340 ms (SD 32.3. P<0.01). In addition, the treatment with thyroxine normalized the TSH level resulted in a decrease of P3 wave latency from 374 +/- 36.3 ms to 343 +/- 16.3 ms (P<0.01). However, in the placebo group such changes were not observed, the latency of P3 being 387 +/- 24.3 ms at the beginning and 379 +/- 36.5 ms at the end of observation period. No significant correlations between P3 wave latency and thyroid parameters were found. In thyroxine treated group a significant improvement in verbal memory (P<0.01), visual memory (P<0.01) and total memory scores (P<0.01) was found, while no changes in these parameters were observed in the placebo group. No significant differences were found in the MMPI test evaluation.

CONCLUSION: SH patients had significantly longer P3 wave latency in ERP examination as compared to healthy individuals which gives evidence for impaired cognitive functions in SH patients. In these patients the normalization of TSH level by thyroxine treatment resulted also in the normalization of P3 wave latency. In addition, also verbal, visual and total memory scores improved significantly with the TSH normalization.