The thyrotropin (TSH) receptor belongs to a family of guanine nucleotide protein-coupled receptors with seven transmembrane-spanning regions joined regulatory together by extracellular and intracellular loops. The cytoplasmic domain comprises three cytoplasmic loops and a cytoplasmic tail that are likely to be important in coupling of the receptor to the guanine nucleotide proteins. To address the question of which portions of the cytoplasmic domain of the TSH receptor are important in this process, we have altered groups of amino acids in the region of the TSH receptor by site-directed mutagenesis. Because of the low affinity of TSH binding to the TSH receptor mutated in the amino terminus of the second cytoplasmic loop and the amino terminus of the cytoplasmic tail, definitive conclusions cannot be made regarding the roles of these regions in signal transduction. However, our data indicate that the first cytoplasmic loop (residues 441-450), the carboxyl-terminal region of the second cytoplasmic loop (residues 528-537), and the carboxyl-terminal (but not the amino-terminal) region of the third cytoplasmic loop (residues 617-625) are important in the ability of the TSH receptor to mediate an increase in intracellular cAMP production. Furthermore, two-thirds of the carboxyl-terminal end of the cytoplasmic tail (residues 709-764; corresponding to the region not conserved between the TSH and lutropin/chorionic gonadotropin receptors) can be removed without functional impairment of the TSH receptor.