99- What are the causes of secondary hypothyroidism in children?
Secondary hypothyroidism may be due to genetic or acquired causes. Genetic
causes include pituitary transcription factor defects (PIT1, PROP1, HESX1,
LHX3, LHX4,) or TSH-β/TRH receptor gene mutations. Sellar–suprasellar
tumors (e.g., craniopharyngioma), infi ltrative disorders of hypothalamo–pitu-
itary region, and CNS insults like head injury, meningitis, and cranial irradia-
tion are acquired causes of secondary hypothyroidism (Fig. 3.18 ).- How to approach a child with secondary hypothyroidism?
In children with secondary hypothyroidism, assessment of other pituitary
hormones including 0800h cortisol, prolactin, and gonadotropins (if the child is
in peripubertal age) should be performed. GH–IGF1 axis should be assessed
after normalization of serum T 4. In addition, imaging of sella is mandatory,
which may reveal sellar–suprasellar mass lesions, structural defects of pituitary
gland (small sella, hypoplastic/absent anterior pituitary, redundant/absent stalk,
and ectopic posterior pituitary bright spot) or can be normal.- How to treat a child with secondary hypothyroidism?
Oral L-thyroxine is the therapy of choice in secondary hypothyroidism. In
children with secondary hypothyroidism and coexisting glucocorticoid defi -
ciency, glucocorticoid replacement should be initiated prior to L-thyroxine
therapy, to prevent the risk of adrenal crisis. L-Thyroxine requirement is usu-
ally lower in secondary hypothyroidism, as TSH-independent thyroid hor-
mone biosynthesis continues in these patients. However, requirement of
L-thyroxine increases with concomitant growth hormone therapy (as GH ther-
apy increases T 4 to T 3 conversion and reduces TSH as a result of increaseda bbFig. 3.18 ( a ) A 6-year-old child with multiple pituitary hormone defi ciency. Note frontal bossing,
midfacial hypoplasia, and cherubic face suggestive of GH defi ciency. ( b ) Sagittal MRI T1-weighted
image showing small sella, absent pituitary ( red arrow ) and stalk, and ectopic posterior pituitary
bright spot ( arrow head , MRI tetrad) suggestive of panhypopituitarism due to pituitary transcrip-
tion factor defect
3 Thyroid Disorders in Children