Improvement in subclinical cognitive dysfunction with thyroxine therapy in hypothyroidism: A study from tertiary care...

To evaluate the effect of hypothyroidism (both overt and subclinical) on cognitive function using latencies of P300 auditory evoked potentials (AEPs). P300 latency suggests that shorter latency times are related to better cognitive performance. P300 latencies were also done after thyroxine replacement to see the effect of treatment on cognitive function.

Biochemically proven new onset cases with hypothyroidism (overt and subclinical) were enrolled into the study, AEPs of these two groups when compared with matched controls. After detailed history and physical examination, P300 potentials were recorded at two points Cz and Pz (Cz: On the midline of the head at the vertex, Pz: On the midline of the head between the vertex and occipital protuberance) using a Nicolet Viking Select neuro diagnostic system version 10.0. The study was done in electrophysiology lab in Osmania Medical College.

A patient characteristics of both cases and controls were comparable. The cases consisted of two groups, overt hypothyroid cases 24, mean thyroid stimulating hormone (TSH) values in them was 94, subclinical cases 21 in whom mean TSH value was 12.3. Mean P300 latencies of all cases at Cz was 342.42 ± 29.5 ms, and at Pz was 345.4 ± 30 ms. Mean P300 latencies of controls at Cz was 296.4 ± 34 ms and at Pz was 297.9 ± 33 ms (difference in P < 0.001). Mean P300 values in overt cases were 362.6 ± 32.9 ms at Cz, and at Pz it was 362.5 ± 33.9 ms. Mean P300 values in subclinical cases were 319.3 ± 30.9 ms at Cz, and at Pz it was 316.4 ± 27.9 ms. P300 values in overt cases were highly significant as compared to controls, and P300 values in the subclinical cases versus controls were also significant (P < 0.001).

P300 latency prolongation in both clinical and subclinical hypothyroid cases shows that cognitive function is affected adversely in hypothyroidism including the subclinical hypothyroid cases. Larger studies evaluating the effect of subclinical hypothyroidism on cognitive function are needed with objective means such as the AEPs P300.