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Context: Thyroglobulin (Tg), encoded by TG, is essential for thyroid hormone synthesis. TG defects result in congenital hypothyroidism (CH). Most reported patients were born before the introduction of newborn screening (NBS)., Objective: We aimed to clarify the phenotypic features of patients with TG defects diagnosed and treated since the neonatal period., Methods: We screened 1061 patients with CH for 13 CH-related genes and identified 30 patients with TG defects. One patient was diagnosed due to hypothyroidism-related symptoms and the rest were diagnosed via NBS. Patients were divided into 2 groups according to their genotypes, and clinical characteristics were compared. We evaluated the functionality of the 7 missense variants using HEK293 cells., Results: Twenty-seven rare TG variants were detected, including 15 nonsense, 3 frameshift, 2 splice-site, and 7 missense variants. Patients were divided into 2 groups: 13 patients with biallelic truncating variants and 17 patients with monoallelic/biallelic missense variants. Patients with missense variants were more likely to develop thyroid enlargement with thyrotropin stimulation than patients with biallelic truncating variants. Patients with biallelic truncating variants invariably required full hormone replacement, whereas patients with missense variants required variable doses of levothyroxine. Loss of function of the 7 missense variants was confirmed in vitro., Conclusion: To our knowledge, this is the largest investigation on the clinical presentation of TG defects diagnosed in the neonatal period. Patients with missense variants showed relatively mild hypothyroidism with compensative goiter. Patients with only truncating variants showed minimal or no compensative goiter and required full hormone replacement., (© The Author(s) 2024. Published by Oxford University Press on behalf of the Endocrine Society. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

SLC26A4 is a known iodide transporter, and is localized at the apical membrane of thyrocytes. Previously, we reported that SLC26A7 is also involved in iodide transport and that Slc26a7 is a novel causative gene for congenital hypothyroidism. However, its detailed role in vivo remains to be elucidated. We generated mice that were deficient in Slc26a7 and Slc26a4 to delineate differences and associations in their roles in iodide transport. Slc26a7 -/- mice showed goitrous congenital hypothyroidism and mild growth failure on a normal diet. Slc26a7 -/- mice with a low iodine environment showed marked growth failure. In contrast, Slc26a4 -/- mice showed no growth failure and hypothyroidism in the same low iodine environment. Double-deficient mice showed more severe growth failure than Slc26a7 -/- mice. RNA-seq analysis revealed that the number of differentially expressed genes (DEGs) in Slc26a7 -/- mice was significantly higher than that in Slc26a4 -/- mice. These indicate that SLC26A7 is more strongly involved in iodide transport and the maintenance of thyroid function than SLC26A4., (© 2022. The Author(s).)

Objectives Congenital hypothyroidism (CH) is the most common congenital endocrine disorder. Recent advances in genetic testing have revealed its causative mutations in some CH patients. However, the underlying etiology remains unknown in most patients. This study aimed to perform clinical and genetic investigation in Japanese CH patients to uncover genotype-phenotype correlations. Methods We enrolled 136 Japanese patients with transient or permanent CH between April 2015 and March 2017, and performed next-generation sequencing of 19 genes implicated in CH. Results We identified potentially pathogenic bi-allelic variants in DUOX2, TSHR, and TPO in 19, 5, and 1 patient, respectively (autosomal recessive), and a potentially pathogenic mono-allelic variant in NKX2-1 (autosomal dominant) in 1 patient. Molecular genetic diagnosis was highly suggested in 26 patients (19%) from 23 families. We also detected a potentially pathogenic mono-allelic variant in five recessive genes (DUOX2, TSHR, TG, DUOXA2, and TPO) in 31 unrelated patients (23%), although the pathogenicity of these variants remains inconclusive. Patients with bi-allelic DUOX2 variants showed a more severe clinical presentation in infancy than those with bi-allelic TSHR variants. However, this trend reversed beyond infancy. There were no statistical differences in initial thyroid stimulating hormone, free thyroxine, thyroglobulin, and levothyroxine dose as of March 2017 between patients with bi-allelic and mono-allelic DUOX2 variants. Conclusions The prevalence of potentially-pathogenic variants in Japanese CH patients was similar to that found by previous reports. Our study demonstrates a genotype-phenotype correlation in Japanese CH patients.

Iodide transport and storage in the thyroid follicles is crucial for thyroid hormone synthesis. Pendrin, the iodide exporter that transports iodide to thyroid follicles, is responsible for Pendred syndrome, a disorder characterized by congenital hypothyroidism and hearing loss. However, thyroid hormone levels are basically normal in patients with Pendred syndrome, indicating the presence of another unknown iodide transporter. Here, we show that SLC26A7 is a novel iodide transporter in the thyroid. We observe that SLC26A7 is specifically expressed in normal thyroid tissues and demonstrate its function in iodide transport. Using whole-exome sequencing, we also find a homozygous nonsense mutation in SLC26A7 (c.1498 C > T; p.Gln500Ter) in two siblings with congenital goitrous hypothyroidism. The mutated SLC26A7 protein shows an abnormal cytoplasmic localisation and lacks the iodide transport function. These results reveal that SLC26A7 functions as a novel iodide transporter in the thyroid and its dysfunction affects thyroid hormonogenesis in humans and causes congenital goitrous hypothyroidism., Competing Interests: Competing interestsThe authors declare no competing interests.

Loss-of-function mutations in the thyrotropin receptor (TSHR) gene were described as a syndrome characterized by thyroid hyposensivity to biologically active TSH, ranging from euthyroid to severe hypothyroidism. In Japanese, a common mutation in the TSHR gene is R450H, which demonstrated moderately impaired receptor function. We studied six subjects of Japanese origin whose major abnormality was persistent hyperthyrotropinemia by genetic sequence analysis of the TSHR gene. Three subjects were homozygous for the R450H mutation, whereas the three remaining subjects were single heterozygous. Homozygous subjects displayed mild hypothyroidism confirmed by moderately elevated basal TSH levels and excessive TSH response to TRH administration. Heterozygous subjects also demonstrated fully or partially compensated hypothyroidism, but less severe than that of homozygous subjects. More frequent involvement of the R450H mutation in the TSHR gene in Japanese was identified. In addition, a good correlation between phenotype and genotype was demonstrated in respect to biochemical analysis and drug dosage. Our observations showed clinical significance of heterozygosity associated with compensated hypothyroidism in spite of only mildly impaired receptor function.