A term newborn of an otherwise healthy mother is noted at day 5 of life to have prolonged unconjugated hyperbilirubinemia, poor feeding, and lethargy. Examination reveals macroglossia, a large posterior fontanelle, umbilical hernia, hypotonia, and mottled, cool skin. The newborn screen, drawn at 18 hours of life using a primary TSH strategy, was reported as normal. Repeat serum testing at day 5 shows a low free T4 with a markedly elevated TSH. What is the most likely diagnosis?