Development of thyroid function between VI-IX month of fetal life in humans.

Well-preserved thyroid glands from 28 fetuses 22-34 weeks of gestational age and from 4 term newborns who survived at most 12 days were examined to study thyroid development in late intrauterine life. Total iodine thyroglobulin (Tg), T4, T3 and rT3 were assayed before and after hydrolysis with pronase. In the same maternity unit the cord-serum of 25 healthy term newborns was also assayed and the urine iodine on the day of delivery was tested in 52 newborns and their mothers. Results are expressed as mean values +/- SD. The thyroid gland weight ratio to body weight for the preterms was 0.063 +/- 0.024; thyroid Tg content 5.9 +/- 4.0 mg/g; total thyroid iodine 41.7 +/- 35.1 micrograms/g; Tg iodination 0.72 +/- 0.37%; hormone quota of Tg iodine 39.9 +/- 12.4. Molar ratios were: T4/Tg 3.3 +/- 1.6, T3/Tg 0.25 +/- 0.14, rT3/Tg 0.072 +/- 0.059. Considerable differences in thyroid iodine content and Tg iodination were observed although the availability of iodine was presumably the same. The high Tg iodination reflects the enhanced uptake of the fetal thyroid. The quantity of iodine involved in hormonogenesis varied greatly among fetuses of the same gestational age, this being in the third term as active as in fully mature thyroids. Hormonogenesis seemed preferentially directed to production of T4 and rT3, which, in the latter case, is higher peripherically. Four preterms with a particularly low level of hormonogenesis were not seen to have deficient levels of thyroid iodine or Tg iodination and the most compromised step was the iodine utilization in the production of hormones.(ABSTRACT TRUNCATED AT 250 WORDS)