Purpose: (1) to evaluate objectively changes in cognitive function and electrophysiologic characteristics associated with hypothyroidism of varying severity and duration in primarily older persons; (2) to determine whether these changes are reversible when a euthyroid state has been attained after treatment with thyroid hormone.
Subjects and methods: We enrolled 54 non-demented hypothyroid patients (31-99, mean 68.6 +/- 16.4 years) with biochemical evidence of hypothyroidism (38 had overt and 14 had minimal hypothyroidism) and 30 euthyroid controls (31-96, mean 63.7 +/- 18.4 years) screened for good general health. We evaluated attention, orientation, memory, learning, visual-spatial abilities, calculation, language, visual scanning, and motor speed using standardized neuropsychological tests. Electrophysiological measures of neurocognitive function included the P300 latency component of the auditory Event-Related Potentials (ERP) and conduction speed from eye to cortex, the P100 latency component of the Patterned Visual-Evoked Potential (PVEP). All patients were studied when hypothyroid. A subset of patients with minimal initial test abnormalities were available to be retested when euthyroid, 5 and 9 months after onset of thyroid replacement therapy.
Results: Hypothyroid patients showed significantly lower scores on the Mini-Mental Status Test (MMS) and on five of 14 neuropsychological tests as compared to controls. The neuropsychological tests affected were copying a cube (visual-spatial function), the Inglis Paired Associates Learning Test-Low and Medium association items (memory and learning), Animal Naming (word fluency/production), and the Trail Making A test (attention, visual scanning and psychomotor function. Hypothyroidism also was associated with longer P100 latencies of PVEPs to 20' checks, but showed no significant differences in PVEP P100 latency to 50' checks, nor in the latency of the auditory ERP component P300. There was a statistically significant correlation between a laboratory index of the severity of hypothyroidism (serum T4) and the Inglis Medium Association items and Animal Naming. There was a statistically significant improvement after 5 months of treatment on three of the timed performance tests that previous studies have shown to be most sensitive to brain dysfunction.
Conclusion: Hypothyroidism in non-demented older adults is associated with impairments in learning, word fluency, visual-spatial abilities, and some aspect of attention, visual scanning, and motor speed. The MMS by itself was sensitive in differentiating hypothyroid patients with cognitive deficits from controls, while electrophysiological measures did not generally differentiate the hypothyroid patients from normal controls. The MMS was not sensitive to treatment effects, but treatment was associated with significant improvements in three of the most sensitive measures of cognitive dysfunction.