Thyroid dysfunction is frequently associated with functional disturbances of the brain such as cognitive impairment1, neurodegenerative disorders2, dementia3, depression, and anxiety4. Transient thyroid dysfunction may also induce neuropsychiatric changes5.
Functional neuroimaging studies suggest a direct association between thyroid and brain activity, therefore, these can provide some clues of underlying mechanisms of thyroid hormones on psychological and physiological effects on the brain.
Specific thyroid hormone enzymes, transporters and receptors are believed to maintain thyroid hormone homeostasis in the brain21–23. Therefore, current laboratory tests for thyroid dysfunction may not accurately measure thyroid hormone status in the brain8,21–23.
Scientists from Korea used functional neuroimaging techniques of positron emission tomography (PET) and single-photon emission computed tomography (SPECT) to study the underlying mechanisms of action of thyroid hormones, the use of sugar and blood flow in the brain. 8
In hypothyroidism, significant decreases in blood sugar metabolism in the brain were identified in 3 parts of the brain [the blue dots]. In hyperthyroidism, a significant decrease was identified in 1 cluster [the red dot]:
They also found a reduced blood flow in one area in hypothyroidism.
Metabolic and perfusion deficits in these area can affect working memory and attention, written word recognition, transient memory retrieval, awareness and imagery of visuospatial input, and priming processes, often compromised in patients with hypothyroidism28.
In a small study thyroid hormone replacement therapy caused a reduction in the somatic complaints and depressive symptoms associated with a restoration of metabolic activity in the brain18.
Another study demonstrated that the blood flow in affected brain areas can be normalised after treating hypothyroidism26.
In patients with hyperthyroidism treatment increased regional activity in these regions and these changes significantly correlated with the anxiety and depressive symptoms13.
These results suggest that thyroid hormones regulate the use of glucose and blood circulation in certain areas of the brain. These findings also demonstrate that the mechanism of neuropsychiatric disturbances in patients with hypothyroidism differs from those with hyperthyroidism.
Pak K, Kim M, Kim K, Kim BH, Kim SJ, Kim IJ. Cerebral glucose metabolism and Cerebral blood flow in thyroid dysfunction: An Activation Likelihood Estimation Meta-analysis. Sci Rep. 2020;10(1):1335. Published 2020 Jan 28. doi:10.1038/s41598-020-58255-5