Congenital Hypothyroidism: Role of Nuclear Medicine

Abstract

Thyroid scintigraphy holds a key place in the etiologic workup of neonatal hypothyroidism. Routine screening for this disorder in maternity hospitals in industrialized countries, for nearly 40 years, has permitted early treatment and thereby helped to prevent its physical and mental complications. Neonatal hypothyroidism affects approximately 1 in 3000 births. The most common causes are abnormal thyroid gland development and defective hormone synthesis by an eutopic thyroid gland. The incidence of the latter has risen in recent years, for reasons that remain unclear. A thorough etiologic workup helps to determine the disease type. Current guidelines recommend thyroid imaging by means of ultrasound and scintigraphy. Ultrasound should be done by a practitioner trained to examine the cervical region of newborns, as the thyroid is very small and must be distinguished from the particular aspect of the "thyroid empty lodge." Ultrasound lacks sensitivity for detecting small ectopic glands but is the gold standard for measuring thyroid dimensions. Scintigraphy provides an etiologic diagnosis in most cases. The two isotopes used in this setting are technetium-99m and iodine-123. The latter isotope gives more contrast and allows the perchlorate discharge test to be performed to detect abnormal iodide organification in the neonate with an eutopic thyroid. If scintigraphy cannot be performed during the neonatal period, a postponed procedure can be achieved after 3 years of age. Close cooperation between the nuclear medicine physician and the pediatric endocrinologist is crucial for timely and optimized scintigraphy.

Congenital Hypothyroidism: A 2020–2021 Consensus Guidelines Update—An ENDO-European Reference Network Initiative Endorsed by the European Society for Pediatric Endocrinology and the European Society for Endocrinology

“…Images are of lower quality than with 123 I. The latter isotope needs later image acquisitions (at 2-3, and 24 hours), but provides more contrast and adds information about organification process, allowing perchlorate discharge testing when the thyroid is eutopic (64,65). Furthermore, it exposes infants to a lower dose of whole-body irradiation than 99m Tc (3-10 lCi/kg vs. 50-250 lCi/kg body weight) (66,67).…”

Clinical genetics of defects in thyroid hormone synthesis

“…In thyroid dyshormonogenesis, thyroid ultrasonography shows eutopic thyroid and depending on the severity of hypothyroidism, the thyroid size may be normal or large. Thyroid scintigraphy using 99m pertechnetate or 123 I reflects NIS expression, because both 99m pertechnetate and 123 I are captured by NIS [47]. When thyroid scintigraphy shows low or absent RAIU, this finding is suggestive of basolateral iodide transport defect ( NIS mutations).…”

“…Normal or high RAIU is suggestive of one of the dyshormonogenesis beyond iodide trapping [5,21]. In neonates and children, 99m pertechnetate is preferred for thyroid scintigraphy, since it results in less radiation exposure [47]. However, 99m pertechnetate is not organified.…”

“…However, 99m pertechnetate is not organified. When thyroid scintigraphy shows normal or high RAIU, 123 I scintigraphy with perchlorate discharge test can detect iodide organification defects [47]. In the perchlorate discharge test, first radioactive iodine is administered, sufficient time is allowed to pass so that the radioactive iodine is captured by the thyroid and then, perchlorate is administered orally.…”

Thyroid scintigraphy in the management of benign thyroid disease

“…Scintigraphy can identify agenesis (absence of uptake), hypoplasia of a gland in situ (with or without hemithyroid), a normal or large gland in situ with or without abnormally high levels of uptake (Figure 4), and an ectopic thyroid at any point along the pathway of the normal embryological descent from the foramen caecum at the base of the tongue to the thyroid cartilage. 46,47 Combining scintigraphy and thyroid ultrasound in an individual patient helps to improve diagnostic accuracy. 46 Thyroid scintigraphy prior to radioiodine ablation In addition to aiding the differentiation of the various aetiologies of hyperthyroidism, thyroid scintigraphy and radioiodine uptake (RAIU) is indicated prior to treatment with iodine -131.…”