Quantitative [ 1 2 5 I]protein G-based immunohistochemistry was used to map the distribution of β 1 thyroid hormone receptor (TRβ 1 ) in normal and thyroidectomized adult rat brain, using a previously characterized polyclonal antibody. The distribution of TRβ 1 -like immunoreactivity in normal brain was largely but not perfectly concordant with previous accounts of TRβ 1 mRNA distribution in rat brain. Thyroidectomy resulted in increased immunolabeling in most brain regions (mean increase: 14%, range: -4% to +25%), with statistically significant effects being observed in 9 of the 36 brain regions examined. Brain regions showing the most pronounced effects included the habenular nucleus (+22%), the oriens layer of the hippocampal CA3 region (+24%), and the lateral geniculate nucleus of the thalamus (+23%). These results demonstrate that the TRβ 1 protein in brain is capable of plastic changes in response to adult-onset alterations in TH levels. The observed pattern of brain regional receptor changes following thyroidectomy may provide clues for functional effects of thyroid function alterations in adults.