DETECTION OF THYROTROPIN BINDING INHIBITORY ACTIVITY IN NEONATAL BLOOD SPOTS

Recent studies have suggested that maternal TSH receptor-blocking anti bodies might be of primary etiological importance in some cases of tra nsient congenital hypothyroidism (CH). Because these antibodies are ex tremely potent, we evaluated the feasibility of identifying babies at risk by using readily available newborn blood spots. Blood spots obtai ned from 84 normal babies (group 1) and from 354 infants whose initial T4 was less than the tenth percentile for the assay and whose TSH was 40 mU/L or more (group 2) were studied without knowledge of the diagn osis. Blood was eluted from spots overnight and evaluated for [I-125]T SH binding inhibition (TBI) to solubilized porcine thyroid membranes. Four spots obtained from 3 group 2 babies, but none of those from the group 1 infants, exhibited TBI activity greater than 3 SD above the no rmal mean (33.9%). Four additional hypothyroxinemic infants whose moth ers had Graves' disease were also negative. Subsequent follow-up revea led that all 3 positive babies had transient CH, and all 3 mothers had primary myxedema. Potent TBI activity was confirmed in the serum of a ll 3 mothers and in the 2 babies in whom it was evaluated at birth. We conclude that newborn blood spots can be used to detect potent matern al TBI activity, and that this identifies a baby likely to have transi ent, rather than permanent, CH. Because of their stability and ease of collection and handling, newborn blood spots should offer a convenien t tool for future studies aimed at defining in more detail the inciden ce and clinical characteristics of this unique syndrome.