TSH desensitization involves decreased coupling of the TSH receptor to the adenylate cyclase regulatory protein, G₈. There is evidence that a desensitization protein in thyroid cells plays a role in this process. The molecular cloning of the human TSH receptor and its stable expression in Chinese hamster ovary (CHO-TSHR) cells allowed us to test whether or not TSH desensitization can occur in a nonthyroidal cell. Similar to human thyroid cells, maximal stimulation of cAMP levels in CHO-TSHR cells was attained after 30–60 min of exposure to bovine TSH. Unlike in human thyroid cells, however, preincubation of CHO-TSHR cells with TSH for 12–16 h did not decrease the subsequent cAMP response to a 1-h pulse of TSH stimulation. That is, the human TSH receptor in CHO-TSHR cells does not undergo functional desensitization. Scatchard plot analysis of specific TSH binding to the CHO-TSHR cells revealed high and low affinity sites (Ka of 1.8 ± 0.4 × 10⁹ M^-1 and 1.4 ± 0.3 × 10^-7 M^-1, respectively), with approximately 105 TSH receptors per cell. This is 10- to 100-fold greater than the number of TSH receptors estimated to be present on human thyroid cells. Untransfected CHO cells exhibited only the low affinity binding site. Prior exposure of CHO-TSHR cells to bovine TSH or to (Bu)₂cAMP for periods up to 24 h did not reduce [^l25I]TSH binding to these cells. In summary, desensitization of the adenylate cyclase response to TSH stimulation does not occur in nonthyroidal cells expressing a human TSH receptor with normal functional and TSH binding characteristics. These data support the concept that a cell-specific protein may be involved in homologous TSH desensitization. (Endocrinology127:1240–1244,1990)