We report an infant with abnormally elevated levels of TSH determined in the Maryland State Laboratory for Neonatal Thyroid Screening, but normal levels in three other laboratories. The TSH level in the infant normalized by six months of age. The mother, who had a history of sarcoidosis, also had factitious hyperthyrotropinemia in the Maryland State Laboratory. Gel chromatography and ammonium sulfate precipitation of maternal serum demonstrated that the factor responsible for the factitious hyperthyrotropinemia was an immunoglobulin G. Maternal TSH levels in the Maryland State Laboratory were normalized by treatment of serum with polyethylene glycol. However, protein electrophoresis, immunoglobulin levels and immunofixation electrophoresis were all normal. We conclude that a subclass of immunoglobulins G, probably resulting from sarcoidosis, interfered with the precipitation of the TSH-antibody complex in the TSH radioimmunoassay of the Maryland State Laboratory.