Congenital hypothyroidism

A third of maternal T4 crosses the placenta and influences fetal development, especially the brain. After birth, there is an acute release of TSH in newborn; with peak serum concentrations in 30min in full term infants; followed by a gradual decline within the next 5 days to less than 10mU/L.

Congenital Hypothyroidism results mostly from thyroid dysgenesis (hypoplastic or ectopic thyroid tissue). A transient form of neonatal hypothyroidism can result from trans-placental passage of maternal TRAb (Thyroid Receptor Blocking Antibodies) antibodies.

Other rare causes include thyrotropin deficiency, defects in thyroid hormone transport, defective synthesis of thyroxine (dyshormogenesis), defect of Iodide transport, iodine deficiency goiter, Thyroid hormone unresponsiveness etc.

Clinical Presentation

– Most babies are asymptomatic at birth, even with complete thyroid gland agenesis; difficult to diagnose clinically within first few weeks of age

– Skin may be cold, dry or scaly; with oedematous short limbs and short fingers

– Sleepy baby with low muscle tone, poor cry; feeding difficulties or incoordination

– Head size may be enlarged due to myxedema of the brain or thick scalp; fontanelle is wide; hair is brittle and scanty

– Eyes are widely spaced, puffy eyes and narrow palpebral fissures; flat bridge of nose

– Mouth is open with large protruding tongue

– Constipation, umbilical hernia

– Bradycardia, cardiomegaly due to small pericardial effusion

– Prolonged physiologic jaundice

Diagnosis

In most developed countries there is universal neonatal screening program (on day 5 in the UK) with heel-prick blood spot on a filter paper (e.g. Guthrie card) to check for elevated TSH

If screening is positive, or clinical suspicion, check by checking for TSH (high) and/or fT4 (low)

Thyroid imaging with Ultrasound and Radionucleotide scanning for demonstration of ectopic thyroid tissue

Management

Early diagnosis and replacement leads to better outcomes.

Without treatment, growth and development are adversely affected in childhood, dentition may be delayed and some develop sensoryneural hearing loss.

Levothyroxine replacement in neonates at starting dose is 10-15mcg/kg once daily

Monitor TSH and fT4, initially every 1 to 2 months, later every 6 months; as guided by local endocrinilogist

Target T4 level in upper end of normal range; and TSH in lower end of normal range