Soru

A $15$-month-old boy is brought to the pediatric clinic for a routine well-child visit. His mother is concerned because he is not yet walking independently and his vocabulary consists of only a single word. He was born at $39$ weeks gestation without perinatal complications, and newborn screening was reported as normal. A review of his growth chart demonstrates that his length has decreased from the $50$ th percentile at $6$ months of age to the $5$ th percentile currently, while his weight has remained steady along the $45$ th percentile. Vital signs include a temperature of $36.8^\circ\text{C}$ ($98.2^\circ\text{F}$), heart rate of $88$/min, and respiratory rate of $22$/min. Physical examination reveals an infant with a persistently open and large anterior fontanelle, generalized hypotonia, and delayed relaxation of the deep tendon reflexes. Laboratory evaluation shows:

Thyroid-stimulating hormone (TSH): $2.1\ \mu\text{U/mL}$ (Normal: $0.5-5.0\ \mu\text{U/mL}$)
Free thyroxine (T4): $0.4\ \text{ng/dL}$ (Normal: $0.9-2.4\ \text{ng/dL}$)

Which of the following is the most appropriate next step in the evaluation of this patient?