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Neonatal Hypothyroxinemia: Effects of Iodine Intake and Premature Birth¹

Unidad de Neonatología, Hospital La Paz, Instituto Nacional de Salud (S.A., J.Q.); and Unidad de Endocrinología Molecular, Instituto de Investigaciones Biomédicas y Facultad de Medicina, Consejo Superior de Investigaciones Científicas and Universidad Autónoma (H.F.E.-M., S.D., M.J.P., G.M.d.E.); and Unidad de Medicina Preventiva, Facultad de Medicina de la Universidad Autónoma (R.H.), Madrid, Spain

Address all correspondence and requests for reprints to: Dr. G. Morreale de Escobar, Instituto de Investigaciones Biomédicas, Facultad de Medicina Universidad Autónoma de Madrid, Arzobispo Morcillo 4, 28029 Madrid, Spain. E-mail gmorreale{at}mvax.fmed.uam.es

We have investigated the effects of iodine (I) intake on urinary I excretion in preterm (PT) babies up to 2 months after birth and its effect on serum T₄, free T₄ (FT₄), T₃, TSH, and thyroglobulin (Tg) levels compared to those in term (T) newborns.

Very premature and very sick infants were in negative I balance for the first weeks after birth. Later, these same infants, as well as the other PT and T newborns, were in positive balance; 75–80% of the ingested I was not accounted for in the urine. The urinary I levels of PT and T neonates cannot be equated to their I intakes.

T₄, FT₄, and T₃ levels in PT and T neonates increased with postmenstrual age, whereas Tg decreased and TSH did not change. Serum FT₄, T₃, Tg, and TSH levels in PT neonates were affected negatively, independently from age, by a low I intake. PT birth also affected T₄, FT₄, and Tg negatively, independently from I intake and postmenstrual age, for at least 6–8 weeks after birth.

Care should be taken to avoid I deficiency in PT neonates. However, even when I intake is adequate, PT newborns are hypothyroxinemic compared to T babies during an important period of brain development. This suggests the possible convenience of interventions that might mimic the intrauterine hormone environment and accelerate maturation.

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