Longitudinal growth, final height and pubertal development are typically normal in male and female individuals with CH in whom L-T4 therapy is maintained as recommended [15, 16]. Pubertal timing and final height are independent of etiology, severity of CH and the start of L-T4 treatment, but girls with a higher initial dose L-T4 (>8 |xg/kg/d) had an earlier onset of puberty .
In contrast to physical signs, the neurodevelopmental outcome of patients with CH largely depends on the early initiation and maintenance of adequate postnatal L-T4 therapy, especially in cases of severe hypothyroidism (T4 < 5 |xg/dl). Despite neonatal screening, 10% of early treated infants with severe hypothyroidism are likely to require special education . Subtle differences in intelligence, school performance and neuropsychological tests in comparison to control individuals, classmates and siblings have been detected in adults with CH despite early L-T4 treatment [18, 19]. While in some studies the severity of CH was correlated with poor developmental outcome, recent observations indicate that delayed and inadequate hormone substitution is a main predictor of clinical outcome [18, 20].
Children with CH may have selective deficits on visual, language, motor, attention and memory abilities . Auditory brainstem evoked potentials were abnormal in 25% of early-treated patients with CH . Recent studies have comprehensively analyzed the temporal patterns of thyroid hormone action in the developing brain . Hypothyroidism in early pregnancy is related to impaired visual attention and processing as well as gross motor abilities.
Exposure to maternal hypothyroxinemia in later pregnancy is linked to an additional risk of subnormal visual skills, including impaired contrast sensitivity, slower response speeds and fine motor deficits . In case that hypothyroidism occurs after birth, language and memory are brain functions predominantly affected.
It has to be considered that syndromatic forms of CH due to functional defects of thyroidal transcription factors or the iodothyronine transporter (table 1) may adversely affect CNS development independent of circulating thyroid hormone levels. The long-term perspective for normal mental and neurologic development is poor for infants with CH not detected by newborn screening. Physical symptoms and growth may normalise when L-T4 treatment is started later but within the first months of life but infants with severe perinatal hypothyroidism frequently have low-to-normal IQ .
Less favorable neurodevelopmental outcome is related to late treatment start, inadequate L-T4 dosage, poor social-economic environment, compliance problems and severity of CH. A better neurodevelopmental outcome was obtained with higher initial L-T4 dose of 11.6 |xg/kg/day [24, 25] and faster time to normalize thyroid function (<2 weeks) . Since thyroid hormone replacement is now more vigorous in achieving early correction than in previous decades, neonates with CH today may have eventually better intellectual and neurological long-term outcomes.
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