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.