Supplementary Material for: A Rare Cause of Thyroid Hormone Abnormalities in an Adolescent: A Case of SBP2 (SECISBP2) Deficiency

Introduction SBP2 (Selenocysteine insertion sequence-binding protein 2, SECISBP2) is essential for selenoprotein synthesis. Selenoproteins play critical roles in cellular redox homeostasis, antioxidant defense, and thyroid hormone metabolism. Deiodinases (DIOs) are selenoenzymes that catalyze the deiodination of iodothyronine and are important for thyroid hormone activation and inactivation. Mutations in SECIS-binding protein 2 (SBP2), which facilitates the incorporation of selenium into selenoproteins, lead to defective production of deiodinases (DIOs). Case Presentation A 13-year-6-month-old female patient presented with constipation and abnormal thyroid function tests. Laboratory tests revealed elevated free T4 (25.5 ng/L), normal TSH (1.2 mIU/L), low free T3 (2.3 ng/L), and decreased serum selenium level (14.37 µg/L). The patient exhibited speech and language delay, along with learning difficulties. These findings suggested SBP2 deficiency. Genetic analysis revealed a previously reported homozygous pathogenic variant, c.358C>T (p.Arg120Ter) in the SECISBP2 gene. Conclusion This case highlights the importance of considering SBP2 deficiency in patients presenting with discordant thyroid function tests—namely elevated free T4, low free T3, and normal TSH—especially when accompanied by neurological or developmental features such as speech delay and learning difficulties.**引言**：硒半胱氨酸插入序列结合蛋白2（Selenocysteine insertion sequence-binding protein 2，SECISBP2，简称SBP2）是硒蛋白合成的必需因子。硒蛋白在细胞氧化还原稳态、抗氧化防御及甲状腺激素代谢中发挥关键作用。脱碘酶（Deiodinases，DIOs）是一类硒酶，可催化甲状腺原氨酸的脱碘反应，对甲状腺激素的活化与失活至关重要。SECIS结合蛋白2（SBP2）可介导硒向硒蛋白中的掺入，当其发生突变时，会导致脱碘酶（DIOs）的合成缺陷。 **病例报告**：一名13岁6月龄的女性患者因便秘与甲状腺功能检测异常就诊。实验室检查结果显示，其游离甲状腺素（free T4）水平升高至25.5 ng/L，促甲状腺激素（thyroid-stimulating hormone，TSH）水平正常，为1.2 mIU/L，游离三碘甲状腺原氨酸（free T3）水平降低至2.3 ng/L，同时血清硒水平下降至14.37 µg/L。该患者存在言语与语言发育迟缓，伴学习困难。上述表现提示为SBP2缺乏症。基因分析显示，SECISBP2基因存在1个既往已报道的纯合致病突变：c.358C>T（p.Arg120Ter）。 **结论**：本病例表明，对于甲状腺功能检测结果不一致的患者——即游离甲状腺素水平升高、游离三碘甲状腺原氨酸水平降低且促甲状腺激素水平正常——尤其是同时伴有言语迟缓、学习困难等神经系统或发育异常表现的患者，需将SBP2缺乏症纳入鉴别诊断范围。