Growth hormone (GH) exerts its multiple actions by binding to a specific receptor (GHR) widely distributed in the organism. It is well established that, in acromegaly, the thyroid gland is larger than normal and that GH increases triiodothyronin concentrations and decreases those of tetraiodothyronin (thyroxine). The aim of the present study was to analyze the presence of GHR and its mRNA in rat and human thyroid gland by Western blot, in situ hybridization techniques, and immunohistochemistry. A band of the expected size for GHR was shown in rat and human thyroid by Western blot. GHR immunoreactivity was found in virtually all follicles. The signal was mainly localized in the cytoplasm, although a nuclear positivity was also found. In situ hybridization techniques demonstrated the presence of GHR messenger RNA in the thyroid gland (cytoplasm of the follicular cells). These results provide direct morphological evidence that GHR is localized in the thyroid gland of mammals and opens up the possibility that GH regulates thyroid cell function directly or via local autocrine or paracrine production of insulin-like growth factor I.