Hormone synthesis and storage in the thyroid of human preterm and term newborns: Effect of thyroxine treatment

Iodine and thyroglobulin concentrations, as well as iodine, T-3, T-4 and si alic acid contents of thyroglobulin, were measured in thyroid glands collec ted postmortem from 42 human premature or term newborns and infants. Three groups were considered: very preterm newborns (24-32 postmenstrual weeks, < 5 days postnatal life), preterm and term newborns (34-41 postmenstrual wee ks, < 5 days postnatal life) and infants (born at term, postnatal age 1-8 m onths). Five very preterm and seven preterm newborns received a daily dose of 10 mu g/kg L-T-4 for at least 3 days. Thyroid weight and sialic acid con tent of thyroglobulin progressed with maturation. Intrathyroidal concentrat ions of iodine and thyroglobulin did not increase significantly before the 42nd week of postmenstrual age. The level of thyroglobulin iodination incre ased during the postnatal life, except in the very preterm neonates. T-4 an d T-3 content of thyroglobulin was directly proportional to its degree of i odination and positively related to its sialic acid content. L-T-4 treatmen t of preterm newborns increased thyroglobulin iodination and T-4-T-3 conten t, without increasing thyroglobulin concentration in the thyroid. It was co ncluded that the storage of thyroglobulin and iodine in the thyroid develop s around term birth. This, associated with the resulting rapid theoretical turnover of the intrathyroidal pool of T-4 in Tg, could be an important fac tor of increased risk of neonatal hypothyroxinemia in the premature infants . The L-T-4 treatment of preterm newborns does not accelerate the maturatio nal process of the thyroid gland. (C) Societe francaise de biochimie et bio logie moleculaire / Elsevier, Paris.