A twin pregnancy with a coexisting complete hydatiform mole and a healthy fetus is rare. Associated with this condition are potentially serious maternal and fetal complications. We describe a case of a woman, 23/40 pregnant, who was diagnosed with a twin pregnancy complicated by a hydatiform mole, vaginal bleeding, hyperthyroidism and preterm labour at 26/40. Her hyperthyroidism was successfully treated with propylthiouracil. The preterm labour resulted in the livebirth of a healthy male infant. The baby developed biochemical hypothyroidism post‐natally. The baby’s thyroid function tests were unexpected, revealing a low T4 and a low‐normal thyroid stimulating hormone. This is the first case reported in the literature to describe an infant’s clinical and biochemical thyroid status after gestational trophoblastic disease complicated by hyperthyroidism.