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This is a clinician-written, evidence-based summary aligned to the USMLE Step 2 CK Content Outline. It is intended for medical students preparing for USMLE Step 2 CK. Management reflects current ACC/AHA, USPSTF, and APA guidelines. Always cross-reference with UpToDate, institutional protocols, and clinical judgment.
The Bottom Line
- Congenital hypothyroidism is usually asymptomatic at birth and is detected by newborn screening
- Classic late features include prolonged jaundice, poor feeding, constipation, hypotonia, large fontanelle, macroglossia, umbilical hernia, and hoarse cry
- Abnormal screen requires urgent confirmatory serum TSH and free T4, but treatment should not be delayed when results are strongly suggestive
- Levothyroxine started within the first 2 weeks of life prevents severe neurodevelopmental impairment
- Newborn screening also detects other serious but initially silent conditions such as PKU, congenital adrenal hyperplasia, hemoglobinopathies, and metabolic disorders
Overview
Congenital hypothyroidism is one of the most important newborn screening diagnoses because delayed treatment causes irreversible intellectual disability. Most infants appear normal initially because of transplacental maternal thyroid hormone. In the United States, newborn screening identifies elevated TSH and/or low T4 depending on state protocol. The Step 2 CK pattern is a newborn or young infant with prolonged jaundice, hypotonia, constipation, poor feeding, and macroglossia, or an asymptomatic infant with abnormal newborn screen requiring immediate confirmatory testing and levothyroxine.
Epidemiology
Congenital hypothyroidism occurs in roughly 1 in 2,000-4,000 newborns. Thyroid dysgenesis is the most common permanent cause; dyshormonogenesis is another cause and is more often familial. Central hypothyroidism is less common and may be missed by TSH-only strategies. Universal newborn screening has transformed outcomes when treatment begins early.
Clinical Features
Symptoms
Often asymptomatic at birth despite significant hormone deficiency
Prolonged neonatal jaundice
Poor feeding, lethargy, excessive sleepiness, or hypothermia
Constipation and poor weight gain
Hoarse cry, decreased activity, or hypotonia
Abnormal newborn screen reported after discharge
Signs
Large anterior/posterior fontanelle and wide sutures
Macroglossia, puffy facies, coarse features if diagnosis delayed
Umbilical hernia
Hypotonia, bradycardia, hypothermia, or poor perfusion in severe disease
Poor linear growth and developmental delay if untreated
Investigations
First-line
Newborn screen reviewIdentify abnormal TSH/T4 result and timing of sample collection; early samples may require repeat testing
Serum TSH and free T4Confirm diagnosis urgently; primary hypothyroidism shows elevated TSH and low free T4
Assess feeding, weight, jaundice, and temperatureDetermine severity and immediate clinical risk
Second-line
Thyroid imagingUltrasound or radionuclide scan can identify dysgenesis or ectopic thyroid but should not delay levothyroxine
Thyroglobulin or antibody testingSelected cases to clarify etiology such as maternal blocking antibodies or dyshormonogenesis
Screen for associated pituitary hormone deficitsIf central hypothyroidism suspected, assess cortisol before thyroid replacement when clinically relevant
Specialist
Pediatric endocrinologyAll confirmed or strongly suspected congenital hypothyroidism warrants urgent endocrinology follow-up
Early intervention/developmental monitoringIf treatment was delayed or developmental concerns are present
1
Immediate response to abnormal screen
- Contact family urgently and obtain confirmatory serum TSH and free T4
- Start levothyroxine promptly for confirmed or strongly suspected congenital hypothyroidism
- Do not wait for imaging or etiologic testing before treatment
2
Levothyroxine therapy
- Use oral levothyroxine tablets crushed and mixed with small amount of breast milk, formula, or water
- Avoid coadministration with soy, iron, calcium, or fiber that may impair absorption
- Monitor TSH and free T4 frequently in infancy and adjust dose to keep thyroid hormone in target range
3
Newborn screening principles
- Most US newborn screens are collected after 24 hours of life; early discharge may require repeat testing
- Premature, low birth weight, transfused, or critically ill infants may need repeat screening
- Positive screens are not diagnostic but require urgent confirmatory testing because many screened disorders are time-critical
4
Developmental prevention
- Treatment within the first 2 weeks is associated with excellent outcomes
- Delayed or inadequate therapy risks intellectual disability, motor delay, hearing problems, and poor growth
- Routine developmental surveillance remains important even after biochemical control
Complications
- Irreversible intellectual disability: The major preventable harm of delayed treatment
- Growth failure: Untreated hypothyroidism impairs linear growth and bone maturation
- Neurodevelopmental delay: Motor, language, and cognitive delays can occur with late or undertreated disease
- Persistent jaundice: Reduced bilirubin conjugation and gut motility can prolong jaundice
- Overtreatment: Excess levothyroxine may cause irritability, tachycardia, poor weight gain, and advanced bone age
USMLE Step 2 CK Exam Tips
- 1Congenital hypothyroidism is usually asymptomatic at birth — newborn screening is the key
- 2Prolonged jaundice + constipation + macroglossia + umbilical hernia = congenital hypothyroidism
- 3Confirm with serum TSH/free T4, but do not delay levothyroxine for imaging
- 4Start levothyroxine within the first 2 weeks to prevent intellectual disability
- 5Thyroid dysgenesis is the most common permanent cause
- 6Crushed levothyroxine tablet is preferred; avoid mixing with soy, iron, or calcium
- 7A normal early newborn exam does not rule out congenital hypothyroidism
practicetest your knowledge on congenital hypothyroidism & newborn screeningApply what you've learnt with USMLE Step 2 CK-style questions from the iatroX Q-Bank — pediatrics and beyond.
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