Factitious increase in thyrotropin in a neonate caused by a maternally transmitted interfering substance.

Patient 1 was born at 41 weeks of gestation by normal delivery and had a normal birth weight of 3000 g. He had an increased thyrotropin (TSH) on day 3 after routine neonatal screening, using blood spots in the Elegance Neonatal TSH ELISA [TSH, 25.4 mIU/L; upper reference limit (URL), 13.0 mIU/L]. As per the standard newborn screening procedure (1)(2), the neonatal screening test was repeated on day 17, and although lower than the previous measurement, the TSH was again above the URL (TSH, 19.6 mIU/L; URL, 10.0 mIU/L). On day 21 of life, the child was euthyroid, had thrived since birth with a current weight of 3600 g, and was feeding well. No jaundice, palpable goiter, or other abnormalities were detectable. His mother had not been given iodine or other medications. She had no family history of thyroid disorder and was euthyroid on history and examination. However, both mother and child had markedly increased TSH values as measured by the laboratory Dade Behring Dimension TSH assay (mother’s TSH, 35.0 mIU/L; reference interval, 0.3–5.0 mIU/L; patient’s TSH, 60.9 mIU/L; URL, 13.0 mIU/L) but with normal free thyroxine (FT4) and free triiodothyronine (FT …