Endocrine Abstracts

Case history: A 30-year-old Afrocaribbean lady with a history of Graves’ thyrotoxicosis (2012) and subsequent thyroidectomy (2013) presented to the antenatal clinic in January 2020 at 9+6w gestation. Medication included levothyroxine 125 mg daily. She had a history of neonatal thyrotoxicosis in her first pregnancy in 2017 which required carbimazole therapy due to fetal tachycardia.

Investigations: At booking, thyroid function tests (TFTs) showed a TSH 6.48mU/land free T4 12.8mU/l. TSH Receptor antibodies were very high at 34.24IU/l. Her anomaly scan was normal. A CTG (cardiotocography) showed a rising fetal heart rate (FHR 150-170bpm) at 32w but the fetal US scan showed a normal thyroid gland with no goitre.

Results and treatment: The levothyroxine dose was initially increased from 125 mg OD to 150/175 mg on alternate days in the second trimester which improved her TFTs at 23w (TSH 1.73mU/l, Free T4 15.1mU/l). Due to fetal tachycardia, carbimazole 5 mg OD was added at 32+3w and plans made for twice weekly CTG monitoring. TFTs at 35w showed TSH 0.33mU/land Free T4 15 pmol/lwith FHR of 140-150bpm. The patient went into spontaneous labour at 36+3w. Her infant male (2960g) was admitted to HDU with a heart rate >200bpm and tachypnoea and was treated for possible sepsis with broad spectrum antibiotics. The neonatal TFTs showed TSH <0.01mU/land Free T4 >95.7 pmol/l. TSH Receptor antibodies were elevated at 4.05 IU/l. Propranolol and carbimazole were commenced to good effect and weaned over 2 weeks. At 5 weeks, the weight was 4400g and TFTs normalised (TSH 1.18, Free T4 12.7 pmol/L; TSH receptor antibodies 0.57 IU/l). The mother stopped taking carbimazole at the time of delivery and returned to her normal levothyroxine dose.

Conclusions: Neonatal thyrotoxicosis is rare but more common with a history of maternal Graves’ disease with high antibody titres requiring treatment in pregnancy TSH receptor antibodies may continue to be produced after thyroidectomy and can cross the placenta and lead to neonatal thyrotoxicosis. This is a transient disorder which ameliorates once maternal antibodies are cleared from the circulation but can have a high mortality rate if not recognised and treated promptly. In this case, despite an improved fetal heart rate and reassuring antenatal thyroid ultrasound scan, the neonate still went on to exhibit clinical and biochemical evidence of neonatal thyrotoxicosis. Thus, early measurement of TFTs from cord blood and close monitoring in the first few days of life is crucial.