L.C.K. Low
Department of Paediatrics, The University of Hong Kong, Hong Kong SAR, PR China
Congenital hypothyroidism (CHT) occurs in 1:3000 to 1:4000 live births. Sporadic thyroid dysgenesis is the most common etiology and mutations in transcription factors TTF-1, TTF-2, PAX-8, TSHR and inborn errors of thyroxine synthesis are rare. Iodine deficiency remains a problem worldwide and neonatal serum TSH concentrations have been used as one of the indicators of population iodine intake. Transient neonatal CHT is a problem in countries where there is borderline iodine intake. Borderline iodine intake in pregnant mothers can have an effect on thyroid size and thyroid function in the mother as well as thyroid function in the neonates. A meta-analysis of published cases confirms that the most important independent risk factor for the eventual developmental outcome appears to be the severity of CHT (low T4 and delayed skeletal maturation at diagnosis). Despite early diagnosis and prompt treatment, mild deficits in visuospatial, memory and attention domains persist in adolescence. There is recent evidence that transient primary CHT may not be a benign condition.
Relative to term infants, serum fT4, fT3, TBG and TRH concentraions in premature infants are lower. Transient hypothyroxinaemia is common in premature infants in the first week after birth (85%). Hypothyroxinaemia of prematurity has been shown to be associated with adverse neurodevelopmental outcome. However thyroxine supplement in premature infants has not been shown to improve the developmental outcome. Treatment of VLBW infants with transient or permanent primary or secondary CHT seems justified but the optimal dosage still needs to be defined.