THYROID RESPONSE TO PARTURITION: RELATIONSHIP TO GESTATIONAL AGE (GA) AND THE RESPIRATORY DISTRESS SYNDROME (RDS)

Abstract

Thyroxine (T4, μg/dl), triiodothyronine (T3, ng/dl), reverse triiodothyronine (rT3, ng/dl) and TSH (μU/ml) were measured in cord blood and at 2, 12, 24 and 72 hrs of age in 18 well infants (GA 32 to 41 wks) and in 5 infants with RDS (GA 31-33 wks).

Both T4 and T3 increased after birth. rT3 decreased transiently at 2 hrs and then increased. The increase in T3/rT3 and decrease in T4/T3 ratios at 2 hrs suggests that the initial T3 surge is due, in part, to an increase in peripheral conversion of T4 to T3. In RDS infants TSH was low at 2 hrs and the T4 increase was minimal T3 and T3/rT3 increased while T4/T3 decreased at 2 hrs: at 12 hrs T4/T3 increased and T3/rT3 decreased, suggesting that T4 to T3 conversion occurs at birth but is subsequently inhibited. Conclusions: a) newborn thyroid adaptation matures with increasing GA; b) the initial TSH and T4 surges are reduced in the RDS infant; and c) the RDS infant manifests decreased T4 to T3 conversion after 2 hrs.

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