The basal and TRH stimulated prolactin serum levels were higher than normal in stage IV of endometriosis, but the mean basal serum prolactin concentrations were 12.5, 16.5, 19.5, and 26.5 ng/mL, and those after thyrotropin-releasing hormone (TRH) administration were 88.3, 114.2, 125.3, and 138.8 ng/mL in patients with stages I, II, III, and IV endometriosis, respectively. Here, TRH is linked to endometriosis.