Expression of the thyroid-stimulating hormone receptor in the folliculo-stellate cells of the human anterior pituitary

TSH secretion from the anterior pituitary is mainly regulated by TRH and th yroid hormones. We hypothesized that in addition the pituitary itself could modulate TSH production by sensing its own TSH release, enabling fine-tuni ng of TSH secretion. For such an ultra-short loop control, the pituitary sh ould contain a TSH receptor (TSH-R). To find evidence for this we screened a human pituitary complementary DNA library with a digoxigenin-labeled TSH- R probe and found 2 positive clones of 32,000 plaques. One clone was sequen ced and found to be completely identical to the thyroid TSH-R. Further proo f was obtained by RT-PCR on a human anterior pituitary obtained at autopsy. In situ hybridization and immunohistochemistry confirmed the presence of T SH-R in the anterior pituitary at the messenger ribonucleic acid level as w ell as the protein level. Moreover, double labeling experiments revealed th at TSH-R messenger ribonucleic acid as well as TSH-R protein colocalize wit h major histocompatibility complex class II expression of folliculo-stellat e cells. We conclude that TSH-R is expressed in a subpopulation of follicul o-stellate cells in the human anterior pituitary. This finding suggests ult ra-short loop regulation of TSH secretion. Putative recognition of the pitu itary TSH-R by TSH-R antibodies might have clinical relevance in Graves' di sease.