Hypothyroidism

Overview

Childhood hypothyroidism spans two distinct scenarios: congenital hypothyroidism (CH) - present from birth, causes irreversible neurological damage if untreated; and acquired hypothyroidism - usually autoimmune (Hashimoto's), developing in adolescence with no irreversible neurodevelopmental risk.

Aetiology

Congenital vs acquired hypothyroidism
FeatureCongenital (CH)Acquired (Hashimoto's)
Most common causeThyroid dysgenesis (absent/ectopic gland) - sporadicAutoimmune lymphocytic thyroiditis
Other causeDyshormonogenesis - AR mutation (e.g. thyroid peroxidase); goitre presentCentral CH - pituitary/hypothalamic dysfunction; low TSH + low fT4
AgeBirthAdolescence
Irreversible neuro harmYes - if untreated in first 2 yearsNo

Feature	Congenital (CH)	Acquired (Hashimoto's)
Most common cause	Thyroid dysgenesis (absent/ectopic gland) - sporadic	Autoimmune lymphocytic thyroiditis
Other cause	Dyshormonogenesis - AR mutation (e.g. thyroid peroxidase); goitre present	Central CH - pituitary/hypothalamic dysfunction; low TSH + low fT4
Age	Birth	Adolescence
Irreversible neuro harm	Yes - if untreated in first 2 years	No

Presentation

Most infants with CH are asymptomatic at newborn screening - maternal T4 crosses the placenta and partially protects, so symptoms develop gradually postnatally.

Congenital hypothyroidism - clinical features
Prolonged neonatal jaundice
Macroglossia
Hypotonia
Umbilical hernia / distended abdomen
Hoarse/low cry
Poor feeding and increased sleeping
Constipation
Enlarged anterior fontanelle
Bradycardia
Myxoedema (puffiness)
Goitre - present in dyshormonogenesis only

•Acquired (Hashimoto's): slowing of linear growth (often first sign), cold intolerance, fatigue, dry skin, constipation, delayed puberty, palpable goitre

•Paradoxical precocious puberty or galactorrhoea in younger children - elevated TSH cross-reacts at FSH/LH receptors

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Thyroid hormone deficiency in the first two years of life causes irreversible damage (impaired myelination and neuronal migration). Hypothyroidism developing after this window does NOT cause the same irreversible neurological sequelae.

Investigations

•Newborn heel prick (Guthrie test) - day 5 of life; measures TSH from dried blood spot

•TSH >=20 mU/L on initial sample - refer immediately

•Borderline (8-20 mU/L) - repeat at 7-10 days; if repeat TSH >=8.0 mU/L - report and refer as 'CHT suspected'

•TFTs (serum TSH + free T4) - raised TSH + low fT4 = primary hypothyroidism; low TSH + low fT4 = central hypothyroidism

•Thyroid ultrasound - first-line structural imaging; identifies dysgenesis or goitre

•Radionuclide scan (technetium-99m) - gold standard for ectopic thyroid tissue or agenesis when USS inconclusive

•TPO-Ab / thyroglobulin antibodies - positive in Hashimoto's thyroiditis

•Auditory assessment - sensorineural hearing loss associated with CH

•Bone age X-ray - delayed bone maturation in acquired hypothyroidism with growth failure

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Preterm infants (<32 weeks) may have a falsely reassuring TSH due to hypothalamic-pituitary immaturity. UK guidance recommends a third sample at 7-10 days after the second sample for infants born <32 weeks.

Management

•Congenital hypothyroidism - timing: levothyroxine must start by 14 days if suspected on initial screen; by 21 days if suspected on repeat (borderline) sample

•First-line (CH): oral levothyroxine 10-15 mcg/kg/day (max 50 mcg/day initially) - licensed solutions or tablets only; oral suspensions unreliable

•Target fT4 in upper part of age-appropriate reference range within 2 weeks; normalise TSH within first month

•First-line (acquired): oral levothyroxine at weight-appropriate dose; start low, titrate to normalise TSH

•Monitoring (CH): TFTs and clinical review at ~2 weeks, 4 weeks, 8 weeks, 3, 4, 6, 8, 10, 12 months; more frequent if thyroid agenesis or after dose adjustment

•Transient CH: trial off levothyroxine considered at 2-3 years (after critical neurodevelopmental window); recheck TFTs after stopping

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In adolescents with acquired hypothyroidism, poor medication adherence is a common cause of rising TSH - explore compliance before assuming disease progression.

Complications

•Untreated CH: irreversible neurodevelopmental disability - spasticity, dysarthria, intellectual disability

•Growth: poor linear growth and short stature

•Puberty: delayed puberty (most common in acquired); precocious puberty/galactorrhoea paradoxically in younger children

•Over-replacement: iatrogenic hyperthyroidism - tachycardia, anxiety, poor sleep, accelerated bone maturation, reduced final adult height

•Psychological: poor self-esteem and depression documented even with good biochemical control - warrants active screening