100
somatostatin tone) or estrogen replacement (due to estrogen-mediated increase
in TBG). After optimal replacement with glucocorticoids and/or L-thyroxine,
there may be unmasking of central diabetes insipidus, as both these hormones
counteract the action of antidiuretic hormone. Serum FT 4 should be monitored
in children with secondary hypothyroidism on L-thyroxine therapy and tar-
geted in the upper half of normal reference range. Serum FT 4 is preferred over
total T 4 as concurrent hormone defi ciencies infl uence the circulating TBG lev-
els. The underlying cause of secondary hypothyroidism should be appropri-
ately treated.- A 7 - year - old boy was evaluated for poor scholastic performance and thyroid
function test showed T 4 4 μ g / dl and TSH 2.3 μ IU / ml. Patient was clinically
euthyroid and serum - free T 4 was done which was normal. What is the likely
diagnosis?
Low T 4 but normal FT 4 and TSH suggest the diagnosis of thyroxine-binding
globulin (TBG) defi ciency. TBG is a glycoprotein which binds approximately
70 % of circulating T 4 and 80 % of circulating T 3. TBG defi ciency is inherited
as X-linked recessive disorder, with a prevalence of 1 in 5000 newborns. The
acquired causes of TBG defi ciency include nephrotic syndrome, hepatic fail-
ure, and drugs like glucocorticoids and androgens. However, these patients do
not require L-thyroxine therapy as FT 4 is normal. - What are the causes of hyperthyroidism during infancy and childhood?
During infancy, the causes of hyperthyroidism include transplacental transfer
of TSH receptor-stimulating antibody (in neonates born to mother with Graves’
disease), McCune–Albright syndrome and TSH receptor-activating mutations.
In children, the most c o mmon cause of hyperthyroidism is Graves’s disease
and, rarely, toxic adenoma and toxic multinodular goiter may also result in
hyperthyroidism.- What are the alterations in thyroid function test in a neonate born to a mother
with Graves ’ disease?
Neonates born to a mother with Graves’ disease may be euthyroid, thyrotoxic,
or hypothyroid. Approximately 1–5 % of neonates born to mothers with Graves’
disease have transient neonatal hyperthyroidism due to transplacental transfer
of TSH receptor-stimulating antibody, which may manifest at birth or after few
days (if mother is on antithyroid drugs). Neonatal transient primary hypothy-
roidism can occur as a result of transplacental passage of maternal antithyroid
drugs or TSH receptor-blocking antibody. Rarely, central hypothyroidism can
3 Thyroid Disorders in Children