Elevated thyroid‐stimulating hormone level in a euthyroid neonate caused by macro thyrotropin–IgG complex

UNLABELLED: Elevated thyroid-stimulating hormone (TSH) was discovered by routine neonatal screening in a newborn with no clinical symptoms. Thyroid function tests were repeated and confirmed a high TSH value but normal total thyroxine (T4) and triiodothyronine (T3). However, the mother also had elevated serum TSH with normal levels of T4 and T3. The results suggested a transmitted maternal interfering factor, and no treatment was started while further investigation was performed. Gel filtration chromatography of serum from both the infant and the mother showed a peak TSH with molecular mass consistent with a TSH-IgG complex (macro-TSH). TSH in the infant decreased to a normal level within 8 months in accordance with a normal rate of elimination of maternal IgG, whereas the TSH level of the mother remained high. CONCLUSION: This case suggests that interfering macro-TSH should be considered in a euthyroid neonate with elevated serum TSH and normal T4 and T3 levels to avoid unnecessary treatment.