TSH, via its G-protein-coupled receptor, activates cell growth of both benign and malignant thyroid tumors. G-protein-coupled receptors (GR) kinase 2 (GRK2) has been reported to regulate the TSH receptor but its role in cancer is unknown. To determine a possible function for GRK2 in the growth process of thyroid cancers, we analysed its expression in normal and tumoral thyroid tissues and studied thyroid cancer cell line proliferation after GRK2 overexpression. Thirty one thyroid tissues, including 16 non-medullary thyroid cancers and 15 adjacent normal tissues, were analysed by immunohistochemistry. Five paired tissues were also studied by western blotting for the GRK2 enzymatic activity. Immunohistochemical staining showed an increase in GRK2 in thyroid cancers including papillary, follicular, and anaplastic types, compared with their adjacent normal tissues. Immunoblot analysis and GRK2 enzymatic activity measurement confirmed immunohistochemical study. TSH and TSH in association with insulin or IGF-I stimulated GRK2 protein accumulation in normal human thyroid cells in primary culture. The TSH effect on the GRK2 expression was mimicked by forskolin. After GRK2 overexpression in two poorly differentiated thyroid cell lines, all the clones showed a significant reduction in cell proliferation, ranging from 28 to 65% inhibition compared with vector alone after 96-h culture. In conclusion, thyroid mitogenic factor-stimulated GRK2 accumulation may explain, in part, high GRK2 levels in differentiated carcinoma, because TSH, insulin, or IGF-I is known to be involved in the thyroid cancer progression. Surprisingly, instead of stimulating, GRK2 reduced cell proliferation revealing a new role for this kinase in the growth of thyroid cancers.