Congenital Hypothyroidism
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Direct answer
Congenital hypothyroidism is thyroid hormone deficiency present from birth, most often from thyroid dysgenesis — an absent, ectopic (commonly lingual) or hypoplastic gland — with dyshormonogenesis and iodine deficiency as the other causes. Because clinical signs appear late and subtly, the disease is detected by newborn screening: a heel-prick thyroid-stimulating hormone (TSH) around day three to five. Treatment is oral levothyroxine at about 10-15 micrograms per kilogram per day, started ideally within the first two weeks, because every untreated week costs intelligence.
What you must remember
- Commonest cause: thyroid dysgenesis (agenesis, ectopic gland, hypoplasia) worldwide; inborn errors of hormone synthesis (dyshormonogenesis, often with goitre) and iodine deficiency make up the rest.
- Screening: heel-prick TSH on day 3-5 (cord blood is an alternative in some programmes); a raised screening TSH is confirmed with venous TSH and free T4.
- Clinical clues: prolonged jaundice, constipation, lethargy and poor feeding, hypotonia, macroglossia, umbilical hernia, large posterior fontanelle, mottled cold skin and a hoarse cry.
- Skeletal marker: delayed bone age, classically absent distal femoral and proximal tibial epiphyses on a knee radiograph at term.
- Treatment: levothyroxine 10-15 micrograms per kilogram per day as a single morning dose, monitored with TSH and free T4 at two to four weeks and adjusted as the child grows.
- Prognosis: early continuous treatment preserves normal neurodevelopment; delayed or erratic treatment leaves permanent intellectual disability — this remains a leading preventable cause of it.
- Transient disease: maternal antithyroid drugs, maternal TSH-receptor antibodies and iodine disturbances can cause a temporary form; permanence is re-tested after about three years.
Common confusion
The screening strategy is the commonest confusion: TSH-based screening detects primary hypothyroidism but misses central (pituitary) hypothyroidism, where TSH is low with low free T4 — some programmes use a combined approach. Candidates also mix up sign sets: prolonged jaundice with constipation, umbilical hernia and a big tongue points to congenital hypothyroidism, while jaundice with vomiting, dehydration and hyperkalaemia in a fortnight-old baby suggests congenital adrenal hyperplasia. Remember too that a goitre indicates dyshormonogenesis or iodine deficiency, not dysgenesis — an absent gland cannot enlarge.
Exam-focused takeaway
NEET-PG tests congenital hypothyroidism at three levels: the screening test and its timing, the cause (dysgenesis, with ectopic gland as the classic subtype) and the treatment dose with its urgency. Vignettes show a three-week-old with prolonged jaundice, constipation, a large fontanelle and a protruding tongue, and ask for the next test (TSH with free T4) or the drug and dose (levothyroxine 10-15 micrograms per kilogram per day). Remember the knee radiograph for bone age and the principle that treatment must not wait once screening is strongly positive. "Screening detects all hypothyroidism" and "treatment can await symptoms" are standard false distractors.
Frequently asked questions
Why screen rather than watch for clinical signs?
Signs take weeks to appear, by which time irreversible brain injury may have begun; day 3-5 testing detects disease while the window is open.
What is the commonest cause?
Thyroid dysgenesis — agenesis, ectopic placement or hypoplasia — accounts for most permanent cases, with dyshormonogenesis and iodine deficiency making up the rest.
What dose of levothyroxine is used?
About 10-15 micrograms per kilogram per day as a single daily dose, started within the first two weeks and adjusted against TSH and free T4.
Which radiograph shows delayed bone age at birth?
A knee radiograph: absence of the distal femoral and proximal tibial epiphyses indicates delayed skeletal maturation in a term newborn.
Can congenital hypothyroidism be transient?
Yes — maternal antithyroid drugs, maternal TSH-receptor antibodies and iodine excess or deficiency can produce it; therapy may be withdrawn under supervision after about three years to test permanence.