Pediatric Hypothyroidism: Neonatal TSH Screening, Neurodevelopment, and Nutritional Care

Early intervention preserves neurological integrity during formative infancy:

• Universal Neonatal Screening: Performed between day 3 and 5 of life utilizing dried blood spots collected on filter paper from a heel prick. Elevated TSH identifies congenital hypothyroidism (incidence approx. 1 in 2,500–3,500 live births).
• Congenital Etiology: 85% arises from thyroid dysgenesis (agenesis, hypoplasia, ectopia), with the remaining 15% stemming from inborn errors of hormonogenesis.
• Critical Therapeutic Window: Initiating weight-adjusted levothyroxine replacement (10–15 mcg/kg/day) within the first 14 days of life fully prevents irreversible neurocognitive deficits (cretinism).
• Acquired Hypothyroidism in Older Children: Predominantly caused by autoimmune thyroiditis (Hashimoto's), classically presenting with growth deceleration and lethargy.

Symptomatology varies markedly with physiological maturation:

• Neonatal and Infantile Features: Prolonged unconjugated hyperbilirubinemia (>14 days), enlarged posterior fontanelle (>0.5 cm), hypotonia, macroglossia, hoarse cry, umbilical hernia, hypothermia, feeding difficulties, and stubborn constipation.
• Older Pediatric Presentation: The hallmark clinical indicator is linear growth deceleration (crossing percentiles downward) coupled with relative weight gain. Delayed bone age, late dental eruption, myalgias, and scholastic underperformance warrant immediate thyroid panel testing.

Micronutrient adequacy supports endogenous conversion and pharmacological stability:

• Iodine and Selenium Balance: Iodine supplies the structural substrate for thyroid hormone iodination (WHO: 90 mcg/day infants, 120 mcg/day children). Selenium functions as the core cofactor for iodothyronine deiodinases converting T4 to active T3.
• Iron Repletion: Thyroid peroxidase (TPO) is a heme-dependent enzyme; coexisting iron deficiency blunts levothyroxine therapeutic efficacy.
• Levothyroxine Administration Rules: Administer tablets crushed in plain water or expressed breast milk on an empty stomach at least 30–60 minutes before morning feeding. Never co-administer with soy formula, calcium supplements, or oral iron, which precipitate non-absorbable chelates in the jejunum.
• Goitrogen Management: Cooking cruciferous vegetables volatilizes isothiocyanates, rendering moderate intake metabolically safe.