Management of foetal hyperthyroidism in a mother with autoimmune hypothyroidism: A case report

Foetal hyperthyroidism is mediated by transplacental passage of thyroid stimulating antibodies (TSAbs) and affects mothers with autoimmune (AI) thyroid disease. We report a case of a 33-year-old woman with a history of AI hypothyroidism and raised TSI after 2 stillbirths with suspect foetal hyperthyroidism. At 20.5 gestational weeks (GW) of her third pregnancy, foetal tachycardia and goitre were detected. TSI levels were 30.9mUI/mL. Methimazole (MMI) was started and adjusted based on ultrasound signs (foetal heart rate and thyroid gland vascularisation). The neonate was born at 35GW and cord blood revealed decreased TSH and normal free T4. MMI was started in the neonate at 2 days of life due to the appearance of asymptomatic hyperthyroidism. This case illustrates a rare recurrence of foetal hyperthyroidism in a mother with AI hypothyroidism. Pregestational thyroidectomy, TSAbs determination, early ultrasound diagnosis and foetal therapy helped us to improve obstetric outcomes.