Hydatidiform Mole With Coexisting Fetus and Syndrome of Inappropriate Antidiuretic Hormone Secretion: A Case Report

CONTEXT: Molar pregnancies have been associated with hyperthyroidism and hypertensive disorders. Coexisting molar and fetal pregnancies, which are very rare, have an even higher risk of complications. CASE DESCRIPTION: We describe a case of hyponatremia due to syndrome of inappropriate antidiuretic hormone secretion (SIADH) associated with a molar pregnancy. A 36-year-old patient at 13 weeks gestation with a coexisting molar pregnancy presented with headache, nausea, and vomiting. She was found to have hypertension, hyperthyroidism, and hyponatremia. The hyponatremia was further assessed with an isotonic saline challenge which resulted in a diagnosis of SIADH. The patient underwent dilation and curettage and her hyponatremia resolved. She later developed gestational trophoblastic neoplasia. CONCLUSIONS: A molar pregnancy can present with unusual associated conditions, such as SIADH. Hyponatremia in a patient with molar pregnancy may be mistakenly attributed to other side effects of trophoblastic tissue (hyperthyroidism, pre-eclampsia, or hyperemesis gravidarum). Hyponatremia in a patient with a molar pregnancy warrants evaluation for SIADH.