Background: Neonatal hyperthyroidism is rare, seen in infants of mothers with Graves’ disease (GD), with transplacental transfer of thyroid-stimulating hormone receptor (TSHR) antibodies (TRAbs). We describe a neonate with severe hyperthyroidism due to TRAbs, born to a mother with autoimmune hypothyroidism. Case presentation: A baby boy born preterm at 35 weeks had irritability, tachycardia and proptosis after birth. The mother had autoimmune hypothyroidism, from age 10, with thyroxine replacement and normal thyroid function throughout her pregnancy. She had never been thyrotoxic. There was a family history of Hashimoto’s thyroiditis (HT) and GD. The baby’s thyroid function on day 3 demonstrated gross thyrotoxicosis, TSH<0.01 mIU/L (normal range [NR]<10 mIU/L), free thyroxine (FT4)>77 pmol/L (20–35), free triiodothyronine (FT3) 15.4 pmol/L (4.2–8.3) and TRAb 18.4 IU/L (<1.8). The mother’s TRAb was 24.7 IU/L. Thyrotoxicosis required propranolol and carbimazole (CBZ). Thyroid function normalized within 10 days. The baby was weaned off medication by 7 weeks. He remains euthyroid. Conclusions: We postulate that this mother had co-existing destructive thyroiditis and thyroid-stimulating antibodies (TSAbs) and TSHR blocking antibodies (TBAb), rendering her unable to raise a thyrotoxic response to the TSAbs but with predominant TSAb transmission to her infant. Maternal history of any thyroid disorder may increase the risk of transmission to an infant, requiring a careful clinical assessment of the neonate, with important implications for future pregnancies.