Clinical significance of thyrotropin-binding inhibitor immunoglobulin levels in newborns and their mothers

We investigated whether determination of thyrotropin-binding inhibitor immunoglobulin (TBII) indices in newborns with hyperthyrotropinemia on neonatal screening, or those born to mothers with autoimmune thyroid diseases, and their mothers could predict the newborns' thyroid status. We selected 195 babies born between 1988 and 1996. TBII indices in the sera of these newborns and their mothers were tested at some time from Day 1 to Day 30 after birth, and followed up after that. The subjects were divided into four groups according to the TBII index. Group 1 consisted of 17 pairs of mothers and newborns who both had serum TBII indices greater than or equal to 15%, including four transiently hyperthyroid and 13 transiently hypothyroid newborns. Group 2 comprised 166 pairs who both had a TBII index less than 15%, including one euthyroid and 165 hypothyroid newborns. Eight of the 165 warranted permanent thyroxine therapy due to athyreosis or ectopic thyroid gland. Group 3 consisted of one pair in which the newborn's serum TBII index was 15% or higher but the mother's was less than 15%: the newborn was transiently hypothyroid. Group 4 comprised 11 pairs in which the newborns' serum TBII indices were greater than or equal to 15% but the mothers' were less than 15%. These 11 newborns were all transiently hypothyroid and nine of them warranted transient thyroxine replacement therapy. The results suggest that the long-term outcome of newborns is good when TBII indices are at least 15% in the mother and child because their thyroid dysfuction seems to be related to the transient presence of the transplacental TBII from the mother. When TBII indices in both mother and child are less than 15%, the newborns warrant further investigation due to the likelihood of permanent hypothyroidism.