Evoked Potentials Show Early and Delayed Abnormalities in Children with Congenital Hypothyroidism*

Abstract

To evaluate the effect of congenital hypothyroidism (CH) on nervous system development, we performed evoked potential studies on 7 CH infants at 3-8 weeks of age before treatment and at four months or more after treatment began. All infants were screened using filter paper determination of T4 and TSH, confirmed by serum specimen determinations. These infants had serum TSH concentrations > 100 µU/ml (normal < 7), and the serum T4 range was 4.1-8.5 µg/dl. All had thyroid tissue on ⁹⁹Tc scan: five had ectopic thyroid tissue, and two had a thyroid gland in the normal location. Four older CH children were tested after 3-6 years of treatment.

Brainstem auditory evoked potentials (BAEP) were abnormal in three of the 7 infants and showed bilateral conduction delays in caudal brainstem regions. The BAEP became normal after 6 months of thyroxine treatment. Visual evoked potentials (VEP) were abnormally delayed and had an immature pattern in the four patients tested at four weeks of age. At age 8 weeks, even in untreated patients, the VEP was normal and remained so. Somatosensory evoked potentials (SSEP) were normal at the time of diagnosis. However, seven patients tested after at least five months of therapy had prolonged central conduction times.

We conclude that infants with relatively mild CH (serum T4 values > 4 µg/dl at 3-8 weeks of age) have evidence of delayed visual system maturation that becomes normal even without treatment and of abnormal caudal brainstem development that resolves slowly with replacement therapy. Our results suggest that normal T4 production is necessary to assure normal brain development at a time before diagnosis of CH can be made by neonatal screening. In addition, therapy may not ensure normal shortening of SSEP latency with age.

Key words

Evoked potentials - Congenital hypothyroidism