Transient hypothyroxinemia of prematurity (THOP) is characterised by low levels of serum thyroxine (T4) and tri-iodothyronine (T3), but normal thyroid-stimulating hormone (TSH) levels until up to 6 weeks. THOP is common in preterm infants and may cause neurodevelopmental disabilities at later ages. Iodine is an essential element for thyroid hormone synthesis. An insufficient iodine intake may contribute to hypothyroxinaemia.
Objectives Whether supplementation with iodine alone increases serum free T4 levels in preterm infants. We conducted a study of thyroid hormones in infants of 24–28 weeks gestation. Placebo group: 27 subjects. Iodine supplemented group: 25 subjects. Blood and urine specimens were collected on days 0, 3, 7, 14, 21, 42, 56 and at discharge. Thyroid hormone testing: free T4: equilibrium dialysis, TSH, total T4, total T3: chemiimmunoluminescent assays. The concentration of iodine in formula and milk samples was determined by modification of the chloric acid method (Benotti and Benotti).
Results No differences were found in demographic and birth characteristics between groups. Group effect of postnatal age is significant (p = 0.0001). Compared with the placebo group, free T4 was slightly higher in the iodine group at all times. The only day on which free T4 was significantly higher in the iodine group was day 3 (p = 0.02). Repeated measures analysis showed a significant (p = 0.017) difference over the time of the study. TSH does not seem to differ between two groups; however, TSH was significantly higher in the placebo group on day 56 (p = 0.01).
Conclusion Iodine supplementation may moderately increase the free T4 level in preterm infants.