Benign ovarian thecoma with markedly elevated serum inhibin B levels mimicking adult granulosa cell tumor

•A rare case of markedly elevated inhibin B associated with an ovarian thecoma.•Inhibin B lacks specificity as a tumor marker for adult granulosa cell tumor.•A full differential for elevated Inhibin B is important for improved preoperative counseling. Elevated serum inhibin B is a classic marker of adult granulosa cell tumors. Here we discuss an extremely rare and informative case of elevated inhibin B associated with an ovarian thecoma. A 57 year-old postmenopausal female presented with recurrent bleeding and was found to have an adnexal mass with an elevated serum inhibin B level of 1,915 pg/mL (normal range 10–200 pg/mL). With a preoperative diagnosis of adult granulosa cell tumor, she underwent surgical management for what was ultimately a benign ovarian thecoma. The diagnosis of thecoma was confirmed by a pericellular pattern of reticulin staining and the lack of a FOXL2 mutation by molecular testing. This case demonstrates that inhibin B lacks specificity as a tumor marker for adult granulosa cell tumor, even at very high levels. Knowledge of benign alternative explanations for this finding can facilitate improved preoperative patient counseling. Pertinent literature is reviewed, with an emphasis on proposed hypotheses for inhibin overproduction.