5125 Severe Gestational Thyrotoxicosis Leading to Unwarranted Treatment

Abstract Disclosure: C. Bell: None. L.A. Barbour: None. Introduction: Gestational transient thyrotoxicosis (GTT) is the most common cause of thyrotoxicosis in pregnancy yet is often confused with Graves’. Although usually mild, we present a severe case leading to the unsupported use of anti-thyroid drugs (ATDs), which could have resulted in fetal hypothyroidism if continued. Case: A 30-year-old female G1P0 at 10 wks with a single gestation presented with 4 wks of vomiting, palpitations, and weight loss. She had no history of thyroid disease. She was diaphoretic and ill-appearing and had a non-tender, smooth normal-sized thyroid, a stare without exophthalmos, and a fine tremor. TSH was < 0.01, FT4 5.57 ng/dL (0.89-1.76), and TT3 201 ng/dL (60-181). TPO Ab 51 (< 60). TRAb, TSI Ab, and TgAb were negative. β-hCG level was 334,106 mIU/mL. She was diagnosed with GTT and briefly hospitalized for dehydration but was re-hospitalized 2 wks later with worsening symptoms and 25 lb weight loss. Unexpectedly, her FT4 and TT3 rose to 6.24 ng/dL and 339, respectively. A thyroid ultrasound showed mildly increased vascularity. PTU was started for seronegative Graves’ at 50 mg tid with metoprolol. After 3 days, her FT4 improved to 3.59 ng/dL and TT3 was 176. Further consultation recommended stopping PTU. She was sent home with metoprolol. Her symptoms and FT4 levels improved. FT4 was normal and hCG levels dropped to 54,821 by 18 wks. Discussion: GTT is caused by a transient elevation in T4 due to direct hCG stimulation of the TSH receptor. It is more common with multiple or molar gestations and hyperemesis gravidarum, also mediated by hCG. It typically results in mild thyrotoxicosis, highlighting the uniqueness of this case. Only a couple of case reports exist where GTT presented as severe thyrotoxicosis or thyroid storm and persisted this long. It is critical to distinguish GTT from Graves’ given their different outcomes and treatment. GTT is commonly misdiagnosed as Graves’ as both result in thyrotoxic labs and symptoms and can present in 1st trimester. GTT typically presents at 8-10 wks and lasts until 14-16 wks when hCG levels fall, but Graves’ may also improve by 2nd trimester due to the immune suppression of pregnancy. It is also possible for both to co-exist. hCG levels >100,000 are associated with GTT but due to the variable sialylation of hCG, which changes its affinity to the TSH receptor and half-life, they are unreliable diagnostically. Key clues supporting GTT include no goiter, exophthalmos, or symptoms pre-pregnancy, negative TRAb and TSI, and, notably, a lower T3/T4 ratio compared to Graves’, as seen in this case. To conclude, GTT is often mistaken for Graves’ but inappropriate use of ATDs can result in fetal hypothyroidism and does not improve outcomes. Although the severity of our case is unusual, accurate distinction from Graves’ by endocrinologists is crucial to ensure optimal maternal-fetal outcomes. Presentation: 6/1/2024