Visual abilities at 6 months in preterm infants: impact of thyroid hormone deficiency and neonatal medical morbidity.

BACKGROUND Preterm infants are at risk for neonatal morbidity, transiently reduced thyroid hormone (TH) levels, and impaired visual abilities. To determine the interrelationship between these factors, we measured TH levels in the period ex utero and compared their visual abilities with those of term infants at 6 months (corrected) of age. METHODS The preterm group consisted of 62 infants stratified by gestational age: Group A (23-26 weeks, n = 10), Group B (27-29 weeks, n = 23), Group C (30-32 weeks, n = 19), and Group D (33-35 weeks, n = 10). Controls were 31 healthy full-term infants. In the preterm group, free thyroxine, triiodothyronine, and thyroid-stimulating hormone levels were measured at 2 and 4 weeks of life and 40 weeks postconceptional age. All infants were assessed for visual acuity, contrast sensitivity, and color vision using electrophysiological techniques. RESULTS Compared with controls, the preterm infants demonstrated reduced contrast sensitivity at low temporal frequencies and slower blue-yellow color processing. Groups did not differ from controls in visual acuity. In the preterm group, reduced contrast sensitivity and slow blue-yellow and red-green color vision processing were associated with low TH levels, low gestational age, and several medical morbidities. CONCLUSIONS Our findings signify that some of the weak visual abilities in preterm infants can be accounted for, in part, by their reduced TH levels in the early postnatal period.