Thyrotropin receptor expression in adrenal, kidney, and thymus

Because the thyrotropin receptor (TSHR) has long been considered a thyroid-specific protein, its presence in extrathyroidal tissues has been controversial. In this study, we sought to detect and quantify this potentially low abundance mRNA in various extrathyroidal tissues using liquid hybridization analysis (LHA) and to detect protein with immunohistochemical studies. Strongly positive protected bands, indicating the presence of both intact (2.4 kb) and variant (1.3 kb) TSHr mRNA, were apparent in LHA gel lanes corresponding to normal thyroid, Graves' thyroid, and thymus. Less abundant protected bands of the same sizes were present in lanes corresponding to normal adrenal, and samples from normal kidney were faintly positive. The full-length transcript:variant transcript ratio was approximately 1:1 in all positive tissues. Immunohistochemical analysis of TSHR-like reactivity in paraffin-embedded thymus, adrenal, and kidney revealed specific staining in each of these tissues. No TSHR mRNA or TSHR-like immunoreactivity was detected in samples from several other normal human tissues. We conclude that measurable TSHR mRNA and protein expression is not restricted to the thyroid gland. Further study is warranted to determine whether these extrathyroidal receptors play a role in normal physiology or in disease.