NEW YORK (Reuters Health) - Cognitive impairments in patients with hypothyroidism may be related to a disruption in hippocampal processes, according to Irish researchers.
"These data suggest that a specific deficit in memory rather than a general slowing of thought processing...is present in patients with marked hypothyroidism and to a lesser extent in patients with subclinical hypothyroidism," senior investigator Dr. James Gibney told Reuters Health.
Dr. Gibney, of Adelaide and Meath Hospital, Dublin, and colleagues came to this conclusion after studying 38 patients before and after open-label levothyroxine (LT4) replacement therapy.
Twenty-one patients with overt hypothyroidism and 17 with subclinical hypothyroidism underwent neuropsychological testing at baseline and 3 and 6 months after LT4 replacement. Nineteen controls also completed the tests.
The investigators report that at baseline, overtly hypothyroid patients had deficits in associative and verbal memory, "which rely upon the integrity of the hippocampal and frontal areas." Patients with subclinical disease had impaired spatial and verbal memory. Thyroid-stimulating hormone levels correlated negatively with these deficits.
After LT4 replacement, verbal memory normalized in both groups, the authors report in the October Journal of Clinical Endocrinology and Metabolism. Spatial memory normalized in the subclinical group, but the overt hypothyroidism group had persistent impairments in spatial and associative memory.
Anxiety and depression scores did not correlate with cognitive function, and measures of attention and response inhibition in patients did not differ from controls.
"The memory deficits appear to be specific to hippocampal and frontally driven tasks," continued Dr. Gibney. "Six months of replacement with LT4 resulted in a general improvement in memory but some abnormalities were still detectable in patients with marked hypothyroidism."
J Clin Endocrinol Metab 2009;94:3789-3797.