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- What is the importance of maternal thyroid hormones in fetal brain development?
Thyroid hormones are essential for the fetal brain development, and they help
in neuronal cell differentiation, migration, synaptogenesis, and myelination.
During embryogenesis (fi rst trimester) maternal T 4 is responsible for neuronal
development. During second trimester (12th week) and onward, fetal thyroid
gland starts functioning and gets mature by 20th week of gestation and becomes
the exclusive source of thyroid hormones. In neonates with sporadic congenital
hypothyroidism, maternal T 4 is the exclusive source for thyroid hormones dur-
ing the entire intrauterine period, and hence, these newborns have near-normal
brain development at birth. However, they should be soon replaced (within
6 weeks of life) with L-thyroxine to avoid the decline in cognitive score as the
brain development continues till the age of 3 years. However in neonates with
endemic cretinism, severe maternal T 4 defi ciency (due to severe iodine defi -
ciency) during the fi rst trimester or throughout the entire gestation results in
neurological and myxoedematous cretin, respectively.
What are the physiological alterations in thyroid function during the neonatal
period?
Immediately after birth of a term baby, there is a physiological TSH surge,
which can be as high as 80 μIU/ml. The elevation of TSH occurs in response to
exposure to cold environment after birth. The elevated TSH increases free T 4
within 24–48h, which results in induction of non-shivering thermogenesis.
Thereafter, there is a decline in TSH (which starts after 60 min of birth) and free
T 4 levels, and by second week free T 4 normalizes and TSH falls to <10 μIU/ml.
What is the rationale of neonatal screening for congenital hypothyroidism?
Congenital hypothyroidism (CH) is the most common preventable cause of
intellectual disability. It is a common disorder with a prevalence of about 1 in
2500 live-births, and many newborns are asymptomatic, even with severe T 4
defi ciency (T 4 <3 μg/dl). Delay in diagnosis and initiation of therapy results in
irreversible neurocognitive dysfunction. Further, relatively simple biochemical
test is available for screening, and therapy is inexpensive and highly rewarding.
Neonatal screening for congenital hypothyroidism has also been shown to be
cost effective.- When to screen for congenital hypothyroidism?
Screening for congenital hypothyroidism is recommended in a newborn between
the second and third day of life. This is because, immediately after birth, there
is a neonatal TSH surge, followed by rapid decline in serum TSH levels during
the fi rst 24h of life (serum TSH levels falls to 50 % of peak value by 2h and to
20 % by 24h). Thereafter, there is a gradual fall in serum TSH. Hence, neonatal
screening performed within the fi rst 24h of life frequently yields false-positive
results and is not preferred. However, sampling from cord blood is indicated in3 Thyroid Disorders in Children