A 3-week-old male infant born to non-consanguineous parents presents with poor feeding, lethargy, and constipation. On examination, the structure marked **B** is prominently enlarged and protruding from the mouth. The infant also has coarse facial features, a hoarse cry, and an umbilical hernia. Serum TSH is markedly elevated at 95 mIU/L (normal <5) and free T4 is low at 0.8 ng/dL. What is the most appropriate immediate management?

Explanation

## Why "Initiate levothyroxine 10–15 mcg/kg/day and monitor TSH and free T4 every 2–4 weeks until euthyroid" is right The clinical presentation—macroglossia (structure **B**), coarse facies, hoarse cry, umbilical hernia, poor feeding, lethargy, and constipation—combined with elevated TSH and low free T4 is pathognomonic for congenital hypothyroidism (cretinism). The enlarged tongue (macroglossia) is a classic dysmorphic feature caused by myxedematous infiltration of the tongue in severe congenital hypothyroidism. According to Nelson 21e Ch 558, the standard treatment is levothyroxine at 10–15 mcg/kg/day for newborns, with titration to achieve TSH <5 and free T4 in the upper half of the normal range. Early initiation (ideally within the first 2–4 weeks of life) is critical to prevent intellectual disability and optimize neurodevelopmental outcomes. Regular monitoring every 2–4 weeks is essential during the titration phase. ## Why each distractor is wrong - **"Perform thyroid ultrasound to confirm thyroid agenesis before starting any treatment"**: While thyroid imaging may eventually be useful for determining the etiology (dysgenesis vs. dyshormonogenesis), it must NOT delay treatment initiation. The infant is already symptomatic with biochemical confirmation of hypothyroidism; every day of delay increases the risk of permanent neurological damage. Treatment should begin immediately. - **"Start propylthiouracil to block excess thyroid hormone production"**: This is incorrect because the problem is thyroid hormone *deficiency*, not excess. PTU is used in hyperthyroidism (e.g., Graves' disease), not hypothyroidism. This would worsen the clinical condition. - **"Administer iodine supplementation and recheck TSH in 3 months"**: Iodine supplementation is appropriate for iodine-deficiency cretinism (endemic in parts of India), but this infant's presentation and biochemistry suggest primary thyroid failure (dysgenesis or dyshormonogenesis), not iodine deficiency. Moreover, waiting 3 months without hormone replacement would cause irreversible neurological damage. Levothyroxine replacement is the immediate priority. **High-Yield:** Macroglossia in a hypothyroid newborn is a hallmark of cretinism; early levothyroxine therapy (within 2–4 weeks) prevents intellectual disability—this is why universal newborn screening for TSH is critical. [cite: Nelson 21e Ch 558]