Purpose or Case Report: Congenital hypothyroidism (CH) represents the most frequent neonatal endocrine disorder and remains a key preventable cause of neurodevelopmental delay. Radiologists play a central role in identifying the underlying cause, yet the scintigraphic interpretation of CH can be challenging without a firm grasp of thyroid embryology, physiology, and the biochemical processes governing thyroid hormone synthesis. This educational poster uses data and experience from our large single-centre review of paediatric thyroid scintigraphy - including the largest reported cohort of dual thyroid ectopia - to illustrate how an understanding of thyroid anatomy, embryologic development, and hormone physiology can be directly applied to image interpretation.

We first review the embryologic descent of the thyroid gland from the pharyngeal floor to its final pre-tracheal position, explaining how developmental arrest leads to dysgenesis patterns such as ectopia, hypoplasia, and athyreosis. We then summarise the steps of thyroid hormone synthesis - iodide trapping, organification, coupling, and release - highlighting the roles of NIS, Pendrin, TPO, THOX, and thyroglobulin, and how specific protein defects or inhibitory states (e.g., iodine overload) produce distinctive scintigraphic appearances.

Building on these physiologic principles, we present a structured approach to interpreting thyroid scintigraphy in congenital hypothyroidism, illustrated with representative cases of each aetiological category: thyroid dysgenesis (athyreosis, ectopia, hypoplasia), dyshormonogenesis (trapping and organification defects), and transient hypothyroidism. A practical diagnostic algorithm is proposed that integrates scintigraphic findings with biochemical and ultrasound data, enabling accurate aetiological classification.

The poster also reviews the advantages of thyroid scintigraphy - including its ability to assess functional tissue, distinguish agenesis from ectopia, and identify dual or subclinical foci not visible on ultrasound - emphasising its continuing value in the era of neonatal screening. This educational framework equips radiologists with the embryologic and physiologic context required to interpret thyroid scintigraphy confidently and consistently across the spectrum of congenital hypothyroidism.
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