Effect of Mild Thyrotoxicosis on Performance and Brain Activations in a Working Memory Task

Abstract

Aims: Disturbed levels of thyroid hormones are associated with neuropsychiatric disorders, including memory impairments. The aim of this study was to evaluate effects of mild induced thyrotoxicosis on working memory and its neural correlates.

Methods: Twenty-nine healthy, male subjects with normal thyroid state participated in the study. Functional MRI was acquired during a working memory task (n-back task) before and after ingesting 250 μg L-thyroxin per day for a period of eight weeks. In addition, neuropsychological tests were performed.

Results: In the hyperthyroid condition the subjects showed slower reaction times, but a higher accuracy in the 0-back version of the memory tasks. Fewer differences between euthyroid and hyperthyroid state were seen for the more difficult conditions of the n-back task. FMRI revealed effects of difficulty in the parahippocampal gyrus, supplementary motor area, prefrontal cortex, anterior cingulate cortex, posterior cerebellum, rolandic operculum and insula (p<0.05, FWE corrected). When comparing euthyroid and hyperthyroid condition in relation to task-induced activation, differences of activation were found in the right prefrontal cortex as well as in the right parahippocampal area. In the psychological assessment, the alerting effect in the Attention Network Task (ANT) and four out of five parameters of the auditory verbal learning test (AVLT) showed an increase from euthyroid to hyperthyroid state.

Conclusions: It can be concluded that even a short-term intake of thyroid hormones leads to an activation of brain areas associated with working memory and to an improvement of accuracy of working memory tasks.

Fig 1

(A) Regions of BOLD signal intensity in the whole brain for effect of difficulty level in n-back task. Results for whole brain analysis for all subjects (n = 28), threshold at p<0.05 corrected for FWE. BOLD signal intensity attenuation especially in parahippocampal gyrus, supplementary motor area, dorsolateral prefrontal cortex, rolandic operculum, bilateral anterior cingulate cortex and posteriorcerebellum. The difficulty of the n-back task condition is the difference in difficulty of 0-back (= control task), 1-back (= easier task) and 2-back (= harder task) condition. (B) Difficuly level in n-back task and thyroid hormone intake interaction. Results for whole brain analysis with one-way ANOVA in all subjects (n = 28). A cluster in the right parahippocampal region and in the right prefrontal cortex survived the statistical threshold at p<0.005, uncorrected.