Diagnosis of iodide transport defect: do we need to measure the saliva/serum radioactive iodide ratio to diagnose iodide transport defect?

BACKGROUND: Iodide transport defect (ITD) is an infrequent condition associated with congenital dyshormonogenetic goiter due to mutations in the Na(+)/I(-) symporter (NIS) gene transmitted in an autosomal recessive manner. Herein, we describe a patient with ITD and discuss the features important for the diagnosis, focusing on whether or not measuring the saliva/serum radioactive iodide ratio is useful.
SUMMARY: A 42-year-old Japanese man attended our hospital in 2010. At that time, he had been off L-thyroxine for several months. He had no obvious mental retardation. His parents were cousins and his sister also had a goiter. Since thyroid dyshormonogenesis could not be ruled out, thyroid function tests, scintigraphy, and ultrasonography were performed. The results showed marked hypothyroidism with a high thyroglobulin level of 627 ng/mL. The results for thyroglobulin antibody and thyroid peroxidase antibody were both negative. Ultrasonography showed an enlarged thyroid gland. Neither the thyroid nor the salivary gland was visualized by (99m)TcO(4)(-) scintigraphy. Therefore, we performed genetic testing for the NIS gene without measuring the saliva/serum radioactive iodide ratio. A homozygous mutation, T354P, was identified in the NIS gene. On the basis of this finding, we could make the definitive diagnosis of ITD due to an NIS mutation.
CONCLUSIONS: We recommend confirming the presence of the thyroid by ultrasonography of the neck first and then performing (99m)TcO(4)(-) scintigraphy. If neither the salivary gland nor the thyroid is visualized, screening for NIS mutations should be undertaken. This approach obviates the need to undertake measurement of the saliva/serum radioactive iodide ratio to diagnose ITD.