Childhood and congenital hypothyroidism

What is congenital hypothyroidism?¹

Congenital hypothyroidism can be defined as (variable) dysfunction of the hypothalamic–pituitary–thyroid axis present at birth, resulting in insufficient thyroid hormone production and consequent severe-to-mild thyroid hormone deficiency. Congenital hypothyroidism may be caused by:

• Abnormal development or function of the thyroid gland.

• Abnormal development of the hypothalamus and pituitary.

• Impaired thyroid hormone action.

If congenital hypothyroidism is not detected and treated early it is associated with irreversible neurological problems and poor growth.

Some infants develop hypothyroidism after birth. This is thought to represent primary hypothyroidism rather than congenital hypothyroidism. Children with untreated primary hypothyroidism do not experience the irreversible neurological problems that are seen with untreated congenital hypothyroidism.

How common is congenital hypothyroidism? (Epidemiology)

• The prevalence of hypothyroidism due to any cause is thought to range between 0.2 and 5.3% in Europe.²

• The incidence of central congenital hypothyroidism (insufficient stimulation by the pituitary of the thyroid gland) is currently estimated at around 1:13,000 births. Central congenital hypothyroidism may occur in isolation, but in the majority of cases (60%) it is part of combined pituitary hormone deficiencies.³

• Risk factors include advanced maternal age, medication during pregnancy, family history of thyroid disease, low birth weight, preterm birth, twin pregnancy and birth defects.⁴

Causes of congenital hypothyroidism (aetiology)

Congenital hypothyroidism (CH) may be due to defects in the pituitary gland, the thyroid gland or the thyroid hormones themselves.

Thyroid gland defects

• A missing, ectopic or poorly developed thyroid gland. This condition accounts for the majority of cases of CH.

• It is not inherited, so that chances of another sibling being affected are low.

Disorders of thyroid hormone metabolism

• These account for 10% of cases of CH.⁵

• Examples include TSH unresponsiveness and defects in thyroglobulin structure.

• These conditions are usually inherited and so there is a risk that further children may also be affected.

Hypothalamic or pituitary dysfunction

• Hypothalamic-pituitary dysfunction usually occurs with other disorders of pituitary dysfunction - eg, lack of growth hormone.

• Hypothalamic causes include tumours, ischaemic damage or congenital defects.

Transient hypothyroidism

• This is usually related to either maternal medications (eg, carbimazole) or to maternal antibodies. In maternal thyroid disease, IgG auto-antibodies can cross the placenta and block thyroid function in utero; this improves after delivery.

A number of genetic defects have been associated with CH. This includes mutations in the 'paired box gene 8' (PAX8) and the 'dual oxidase 2 gene' (DUOX2). The PAX8 gene is particularly linked to the formation of the kidney and thyroid gland.⁶ The DUOX2 gene encodes an enzyme called dual oxidase 2 which is crucial to the production of thyroid hormones.⁶

Signs and symptoms of congenital hypothyroidism (presentation)

Infants are usually clinically normal at birth, due to the presence of maternal thyroid hormones.

Symptoms

• Feeding difficulties.

• Somnolence.

• Lethargy.

• Low frequency of crying.

• Constipation.

Signs

• Large fontanelles.

• Myxoedema - with coarse features and a large head and oedema of the genitalia and extremities.

• Nasal obstruction.

• Macroglossia.

• Low temperature (often <35°C) with cold and mottled skin on the extremities.

• Jaundice - prolongation of the physiological jaundice.

• Umbilical hernia.

• Hypotonia.

• Hoarse voice.

• Cardiomegaly.

• Bradycardia.

• Pericardial effusion - usually asymptomatic.

• Failure of fusion of distal femoral epiphyses.

• The growing child will have short stature, hypertelorism, depressed bridge of nose, narrow palpebral fissures and swollen eyelids.

• Refractory anaemia.

• A goitre may be present (more likely with dyshormonogenesis, thyroid hormone resistance and transient hypothyroidism).

Other congenital defects may also be present, eg, atrial septal defects or ventricular septal defects.

Infants not treated early may have delayed mental development, learning difficulties and poor co-ordination.

Diagnosis⁷

All babies in the UK are screened at birth using blood taken via a pinprick and analysed for TSH and T4. This is part of the UK Newborn Screening Programme (the blood is also analysed for phenylketonuria, cystic fibrosis and sickle cell disease, medium-chain acyl-CoA dehydrogenase deficiency, maple syrup urine disease, isovaleric acidaemia, glutaric aciduria type 1 and homocystinuria). See also Newborn Screening.

Babies with a TSH concentration of ≥20.0 mU/L in whole blood on the initial screening sample (or on a second sample for preterm babies <32 weeks gestation) have a positive screening result for CH.

Babies with a TSH concentration of between ≥8.0 and <20.0 mU/L on the initial screening sample (or on a second sample for preterm babies <32 weeks gestation) have a borderline screening result for CHT.

On detecting a borderline screening result, take a second sample (or a third sample for preterm babies <32 weeks gestation) 7 to 10 calendar days after the initial sample.

If the TSH concentration is <8.0 mU/L on this second sample (or on the third sample for a preterm baby <32 weeks gestation), the baby has a negative screening result for CHT.

If the TSH concentration is ≥8.0 mU/L on this second sample (or on the third sample for a preterm baby <32 weeks gestation), report and refer the baby as ‘CHT suspected’.

Management of congenital hypothyroidism⁷

The aim of treatment is early detection and early thyroid hormone replacement to ensure that infants do not develop irreversible neurological disability.

• Babies diagnosed with congenital hypothyroidism should start treatment with oral levothyroxine within the following timeframes:

  • Congenital hypothyroidism suspected on initial screening sample: by 14 days of age.

  • Suspected on a second sample that follows a borderline TSH: by 21 days of age.

• The starting dose of oral levothyroxine should be 10 to 15 mcg/kg/day, with a maximum dose of 50 mcg/day. Only licensed solutions and tablets of levothyroxine should be used. Suspensions may be unreliable.

• The aim of treatment is to increase free T4 close to the upper age appropriate reference range within the first 2 weeks of treatment and to normalise the TSH within the first month.

• Free T4 concentrations may exceed the normal reference range at the time of TSH normalisation but significant elevation should be avoided.

• The thyroxine dose may need to be reduced at an early stage if TSH is suppressed or if the baby is showing signs of overtreatment.

• Regular dose adjustments may be required.

• Babies with significant endogenous thyroid hormone production and associated modest TSH elevation may need smaller initial doses of thyroxine than babies with thyroid agenesis.

• Once levothyroxine treatment has been started, check TSH and thyroid hormone concentrations at an appointment with the expert paediatrician or team member at approximately 2 weeks, 4 weeks, 8 weeks, 3 months, 4 months, 6 months, 8 months 10 months and 12 months after treatment is started, and thereafter as indicated.

• More intensive biochemical monitoring is frequently required, particularly in babies with no endogenous thyroid hormone production or following dose adjustment.

• Monitoring should also include growth and developmental milestones.

Adverse sequelae

The main adverse effects are related to the lack of adequate thyroid hormone replacement leading to hypothyroidism, or excessive thyroid hormone replacement leading to hyperthyroidism, manifesting as tachycardia, anxiety and a disturbed sleep pattern.

Prognosis

If CH is detected early in infants and treatment begun, normal development of mental function can occur. If treatment is delayed, spasticity, gait problems and dysarthria and profound mental disability may result.

Poor self-esteem and depression are amongst several factors that lead to a poorer quality of life in patients who have been treated for CH.⁸ A high index of suspicion with careful questioning is required to pick up on these aspects.

Acquired childhood hypothyroidism⁹

The most common cause of childhood hypothyroidism is lymphocytic thyroiditis, also known as Hashimoto's autoimmune thyroiditis.

• This is typically seen in adolescence, but can occur earlier.

• There is a high incidence in children with Turner syndrome and Down's syndrome.

• First signs are slowing of growth (often unrecognised) with other typical signs of hypothyroidism - eg, skin changes, cold intolerance, sleepiness and low energy.

• Typically puberty is delayed, although younger children may have galactorrhoea or precocious puberty.

• One particular issue is poor medication compliance in adolescents, which can lead to apparently unexplained deterioration in thyroid function.¹⁰

The cause may be iatrogenic (eg, treatment for hyperthyroidism). Rarer causes include acute suppurative thyroiditis and subacute non-suppurative thyroiditis (de Quervain's disease).

Subclinical hypothyroidism (SH) in children

This is quite common in children and adolescents. The effects of untreated subclinical/mild hypothyroidism are still not completely defined. In the neonatal period, concern exists about neurocognitive outcome. In children, although there is no clear evidence of alterations in growth or neurocognitive development, subtle cardiovascular abnormalities have been documented.¹¹

See separate article Subclinical Hypothyroidism.

Dr Mary Lowth is an author or the original author of this leaflet.