Abstract Disclosure: J. Hayes dorado: None. M. Calla Ayala: None. A. Colque Arias: None. K. Lanuza Gaite: None. M. Rossell Lopez: None. D. Frias Eid: None. Van Wyc-Grumbach syndrome (VWGS) is a rare presentation of long standing prepubertal hypothyroidism characterized by bilateral multicystic enlarged ovaries, vaginal bleeding and delayed bone age. It appeared that high TSH could produce FSH and LH like activity leading to multiple ovarian cysts. A 7-year-old girl with trisomy 21, presented with vaginal bleeding for 2 days prior to admission. Physical exam revealed bradycardia (68 bbm), apathy, sluggishness, as well as dry skin and eyelid oedema. She had no breast development nor pubic hair.Causes of vaginal bleeding such as trauma, possible abuse, foreign body insertion and urethral prolapse were excluded. There was no purpura or bleeding from other sites.The abdominal sonogram revealed bilateral ovarian cystic masses (V:17.29 cm3 and 16.48 cm3, right and left, respectively). Bone age was delayed almost 3 years behind her chronological age. Laboratory tests revealed TSH 860 μIU/mL, T4 0.71 μg/dL, FSH 8.12 mIU/mL, LH < 0.1 mIU/mL, estradiol 193 pg/mL, thyroid peroxidase Abs 184 IU/mL, thyroglobulin Abs 212 IU/mL, Β-hCG < 0.1mU/mL, confirming severe hypothyroidism. The girl was diagnosed with VWGS due to unrecognized chronic lymphocytic thyroiditis and LT4 treatment (75mug/OD) was initiated. The patient responded with complete remission of symptoms and regression of the ovarian cysts one month after initiation of treatment. It is very important to think of VWGS and investigate for thyroid status during the evaluation of ovarian cysts or isolated premature menarche. Early recognition can eliminate unnecessary surgery to remove ovarian cysts, as appropriate treatment with levothyroxine leads to complete remission of symptoms. Even when the diagnosis of VWGS is confirmed, practitioners must be watchful to consider surgical intervention in the presence of uncontrolled vaginal bleeding, hemodynamic instability, or failure of regression of ovarian cysts with exogenous thyroid hormone replacement. Presentation: Friday, June 16, 2023