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Initial evaluation of infants with confirmed CH should be accomplished promptly
and should require no more than 2–5 days.22,27,49 In the absence of facilities
or funds to conduct scanning or ultrasound studies, treatment should be instituted
as soon as the diagnosis is confirmed and appropriate samples for in-vitro laboratory
testing have been collected. The goal of newborn thyroid screening is the
institution of early, adequate thyroid hormone replacement therapy. While the
critical period of dependency of brain maturation on thyroid hormone extends to
2–3 years of life, the first 6 months are most critical since IQ during this time
in the absence of thyroid hormone may be decreased by 30 points or more.2,3 The
essential hormone analogue for brain maturation is T3, and approximately 70 % of
the T3 in the cerebral cortex is derived via local T4 monodeiodination.50
Thus, the preferred thyroid hormone preparation for treatment is Na-l-thyroxine.
The dosage of T4 should be 10–15 µg/kg/day to normalize the serum T4 level as quickly as possible, preferably within 2 weeks.26,27,51 To guarantee adequate hormone to all infants, it is necessary to maintain the serum T4 or/and free T4 in the upper half of the normal range during therapy. The 97 % upper limit of serum T4 levels in hypothyroid infants often ranges to 130 nmol/L (10 µg/dL) so that the initial target for the total T4 concentration is 130–206 nmol/L (10–16 µg/dL) and for free T4 30–45 pmol/L (2.3–3.5 ng/dL) by immunoassay.27,51 The mean values may remain supraphysiologic for several weeks or months and serum TSH usually is suppressed to the normal range by 2–3 weeks. Fear of untoward CNS effects of transient overtreatment are unwarranted; healthy term infants experience a transient hyperthyroxinemia during the neonatal period with free T4 values in the 50 to 75 pmol/L (3.9–5.8 ng/dL) range by immunoassay.51 Therapy is monitored by regular measurements of T4 or/and free T4 and TSH with dosage adjustments to maintain the T4 and free T4 levels above 130 pmol/L (10 mg/dL) and 30 pmol/L (2.3 ng/dL), respectively during the first year. Thereafter levels should be maintained in the upper half of the normal range and thyroxine dosage monitored to maintain normal growth and bone maturation. |
