7527 Subacute Thyroiditis in First Trimester of Pregnancy: Rare Cause of Thyrotoxicosis

Abstract Disclosure: K. Wilcher: None. A. Gupta: None. Background: Subacute thyroiditis (SAT) is extremely rare in pregnancy with only 8 prior cases reported in the literature. SAT can result in maternal and fetal complications. In this case report, we describe a case of subacute thyroiditis presenting in the first trimester of pregnancy. Case Presentation: A 38-year-old female with past medical history of stillbirth presented with thyrotoxicosis in gestational week 10 of her pregnancy. The patient has a history of stillbirth in gestational week 38 of her first pregnancy requiring cesarean section one year prior to presentation. She was in the process of getting in vitro fertilization (IVF) and had an egg retrieval. During the process, she started on thyroxine 50 mcg daily for blood thyroid-stimulating hormone (TSH) 3.77 mcIU/mL. She became pregnant without the help of IVF and stopped thyroxine. One month later, labs showed low TSH 0.022 mcIU/mL (reference range: 0.4-4.5 mcIU/mL) and high blood free thyroxine (FT4) 1.87 ng/dL (reference range: 0.7-1.8 ng/dL). Repeat labs two weeks later showed low blood TSH 0.012 mcIU/mL, high FT4 2.08 ng/dL, and high blood free triiodothyronine (FT3) 5.2 pg/mL (reference range: 2.3-4.2 pg/mL). She was referred to endocrinology for evaluation. She reported mild intermittent right sided neck pain since diagnosis of pregnancy. She denied any history of viral infection, cough, or fever. She reported symptoms of severe nausea, mild constipation, and mild anxiety. She denied diarrhea, tremors, changes in voice, palpitations, or weight loss. Her family history is significant for hypothyroidism in her father. Physical exam demonstrated a non-tender, symmetric thyroid without enlargement or nodules. Ultrasound showed heterogenous thyroid with an ill-defined heterogenous area in the right lobe appearing pseudo-nodule, and increased vascularity. At this time, the patient was diagnosed with mild gestational hyperthyroidism with a plan to monitor thyroid hormone levels. Repeat labs two weeks later showed low blood TSH 0.005 mcIU/mL, high FT4 2.35 ng/dL, and high FT3 6.4 pg/mL. TSH-receptor antibodies, thyroid stimulating immunoglobulin, and thyroid peroxidase antibodies were normal on testing. In gestational week 16, labs showed normal TSH 2.76 mcIU/mL, low FT4 0.56 ng/dL, and low FT3 2.1 pg/mL. At that time, she started on levothyroxine 75 mcg daily and was diagnosed with subacute thyroiditis. Follow up labs in gestational weeks 18, 20, and 24 showed improvement in TSH and FT4 on thyroid hormone supplementation. Conclusion: This case report discusses the findings of SAT in first trimester pregnancy. Currently, she is in gestational week 28 and still requiring thyroid hormone supplementation. SAT in pregnancy is very rare. This case report significantly contributes to the literature and current knowledge of the prevalence, presentation, and disease process of subacute thyroiditis in first trimester pregnancy. Presentation: 6/2/2024