• Three patients demonstrated transient neonatal hypothyroidism, presumably secondary to maternally derived thyrotropin (TSH)–blocking antibodies. Although transient, this disorder might not have been benign in the first child, who exhibited significant developmental delay. A thyroid scan was not helpful in making this diagnosis. Although uncommon, this disorder should be suspected in infants with a maternal history of autoimmune autoimmune thyroid disease, multiple siblings with congenital hypothyroidism, or a clinical course characterized by continually suppressed TSH levels, despite low doses of levothyroxine sodium replacement. Measurement of TSH-blocking antibodies may be used in the diagnosis of transient neonatal hypothyroidism at birth and is becoming more readily available from reference laboratories. Once diagnosed, the patient may then be prepared for monitored withdrawal of levothyroxine replacement therapy at 2 to 3 years of age and Will not be committed to lifelong replacement therapy.