Usefulness of serum thyrotropin-binding inhibitory index measurements in infantile hypothyroidism. Relationship to serum thyrotropin concentrations
D. I. Shulman, J. A. Strzelecki, B. B. Bercu and A. W. Root
Department of Pediatrics, University of South Florida College of Medicine, Tampa.
Transplacental passage of thyrotropin (TSH)-binding inhibitory
immunoglobulins may result in transient congenital hypothyroidism. We
measured serum TSH-binding inhibitory index (TBII) in 11 infants with
abnormal screening findings using a commercially available kit. Two of the
infants, who were siblings, had markedly elevated TBII values (90% and
100%, respectively), as did their mother (89%, 100%), and had a clinical
course consistent with transient antibody-mediated hypothyroidism. Four
other infants had a borderline or mildly elevated TBII that was not present
in maternal serum, suggesting that endogenous TSH was being measured in
this assay. The TBII was measured in the sera of 18 additional children
with primary hypothyroidism and in human TSH standards from 25 to 2000
mU/L. Increasing concentrations of TSH were associated with a linear
increase in TBII. Measurement of TBII by this method may identify infants
with transient antibody-mediated hypothyroidism, although simultaneous
assessment of maternal serum is necessary.
