Maternal thyroid dysfunction can negatively influence fetal and/or neonatal thyroid hormone homeostasis. Autoantibodies associated with autoimmune thyroid disease can cross the placenta. TSH receptor antibodies (TRAbs) can either stimulate or block the TSH receptor, and both types of antibodies can be present in the same person. TRAbs are the most important antibodies in Graves' disease but are also found in a percentage of women with Hashimoto disease. Properties of the dominant TRAb type (stimulating or blocking) will generally dictate the clinical picture. We describe a rare case of neonatal hypothyroidism followed by hyperthyroidism caused by maternal TRAbs, associated with Hashimoto disease. In contrast to similar cases, the mother was not treated with antithyroid drugs, providing evidence for the gradually changing balance between blocking and stimulating TRAbs after birth and their different effects on neonatal thyroid function. This case highlights the need for regular thyroid function tests in neonates with high TRAb titers until maternal antibodies are cleared.