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Q1: Epidemiology and why we screen (2 min)
"What is congenital hypothyroidism, why does it matter, and how common is it?"
- Thyroid-hormone deficiency present at birth; most common preventable cause of intellectual disability worldwide.
- Incidence 1 in 2000 to 4000 live births; female-to-male about 2 to 1.
- Most neonates are asymptomatic at birth (maternal T4 crosses placenta) — clinical detection is too late to prevent brain damage, so universal newborn screening is essential. CH was the first condition screened universally (1970s).
Q2: Aetiology and pathophysiology (3 min)
"Classify the causes and explain the molecular mechanism of brain injury."
- Primary permanent: thyroid dysgenesis (~80%) — ectopic (commonest), agenesis, hypoplasia; sporadic. Dyshormonogenesis (~15 to 20%) — autosomal recessive, goitrous; TPO commonest; Pendred (SLC26A4) = goitre + sensorineural deafness.
- Transient (~10%): maternal TSH-receptor blocking antibodies, maternal antithyroid drugs, iodine excess (Wolff-Chaikoff — contrast/antiseptics) or deficiency, prematurity.
- Central (rare): low free T4 with low or inappropriately normal TSH — missed by TSH-only screening.
- Brain injury mechanism: T3 (from local D2 deiodination of T4) drives myelination, neuronal migration, synaptogenesis — most critical in the first 3 years; deficiency → impaired myelination, reduced synapse density → irreversible intellectual disability.
Q3: Diagnosis and investigations (2 min)
"How do you confirm and classify a positive newborn screen?"
- Screen: dried-blood-spot TSH on heel-prick, day 3 to 7; TSH cut-off program-specific (20 to 60 mU/L), should be age-adjusted.
- Confirm: venous TSH (high) + free T4 (low). Treat immediately — do not wait for scans.
- Classify (after treatment started): thyroid ultrasound, scintigraphy (technetium-99m / iodine-123) to localise the gland; maternal TSH-receptor antibodies; bone age; genetic tests if dyshormonogenesis/syndromic. Always exclude adrenal insufficiency before giving T4 in central CH.
Q4: Management and prognosis (3 min)
"What is the drug, dose, target, and monitoring? When do you re-evaluate permanence?"
- Levothyroxine 10 to 15 mcg/kg/day oral, once daily, started immediately on diagnosis — treat first, investigate later.
- Administration: morning, empty stomach, 30 to 60 min before feed; separate from calcium, iron, soy, antacids.
- Target: free T4 upper half of range within 2 weeks; TSH 0.5 to 5 mU/L.
- Monitor: at 2 and 4 weeks, then every 1 to 2 months in year 1, every 2 to 3 months in year 2, 3 to 12 months thereafter, 4 weeks after any dose change.
- Re-evaluate at age 3: trial off treatment 4 to 6 weeks → rising TSH = permanent (lifelong); normal = transient.
- Prognosis: treatment within first 2 weeks → near-normal IQ; outcome proportional to severity, age at treatment, starting dose, and adherence. Increase L-T4 by 25 to 30% in pregnancy.
Probe questions (examiner may push)
- "What bedside sign in a neonate most strongly suggests CH?" — large posterior fontanelle / prolonged jaundice / macroglossia / umbilical hernia.
- "Name three syndromic forms of dysgenesis." — Bamforth-Lazarus (FOXE1), brain-lung-thyroid (NKX2-1), PAX8 with renal anomalies.
- "A Down syndrome infant — what is your thyroid plan?" — screen at birth and at least annually (Down syndrome is both a differential and a true risk factor for CH).
- "Why might TSH screening miss central CH?" — TSH is low/inappropriately normal; need free T4 and pituitary work-up.