The pleiotropic function of thyroid hormones (TH) is mediated by an organ specific
expression of thyroid hormone transporters, deiodinases and TH receptors. In a series
of studies we used the model of an experimentally induced hyper- or hypothyroidism
in human volunteers to delineate TH action on the brain. A battery of neuropsychological
testing paradigms was employed and complemented by structural and functional multimodal
neuroimaging. Experimentally induced mild thyrotoxicosis for 6 weeks was associated
with changes in brain structure (determined with voxel-based morphometry), resting
state functional connectivity, and task-related functional activation in a working
memory paradigm. Partial withdrawal of TH replacement in patients without thyroid
(subclinical hypothyroidism) likewise lead to changes on multiple functional and structural
brain measures. Importantly, the series of studies reviewed here identified the cerebellum
as one crucial site of action.
