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.