A 6-month-old infant with Down syndrome (trisomy 21) is brought to the pediatric clinic for routine follow-up. The mother reports that the child has been gaining weight appropriately and meeting developmental milestones with early intervention. On examination, the infant is alert, with normal vital signs and no cardiac murmur on auscultation. However, the mother mentions that the child seems to tire easily during feeding and has mild respiratory symptoms. Thyroid function tests done at 4 weeks were normal. What is the most appropriate next step in management?