Endocrine Abstracts

JOINT3557

Background: Biallelic pathogenic variants in the Thyroglobulin(TG) gene have a well-established correlation with congenital hypothyroidism (CH) due to thyroid dyshormonogenesis (TDH), frequently associated with congenital goiter. The phenotypic spectrum is highly variable. Here, we describe a novel homozygous intronic TG variant in a family with recurrent congenital goiter and demonstrate its pathogenic impact through its transcript analysis.

Case Report: The proband, a 15.5-year-old male, was referred for a thyroid nodule. He had a prenatal diagnosis of goiter and was started on L- thyroxine postnatally due to CH. Born at term (3100 g) via spontaneous delivery, he required intensive care due to respiratory distress. He has been on L-thyroxine since infancy, with age-appropriate neurodevelopment. His parents were first-degree cousins, and his mother had a previous pregnancy complicated by fetal goiter. At presentation, his height and weight were 164.5 cm(-1.1SDS), and 73.5kg(0.8SDS), respectively. The thyroid gland was enlarged, and systemic examination was unremarkable. Thyroid ultrasound showed diffuse heterogeneity with fine septations and a 9 mm solid nodule. During follow-up, the iso-hypoechoic nodule increased to 14×9.5 mm. Fine needle aspiration biopsy (FNAB) initially indicated atypia of undetermined significance (Bethesda 3), later raising suspicion for follicular/Hurthle cell neoplasm (Bethesda 4). Right lobectomy and isthmectomy revealed adenomatoid hyperplasia with oncocytic changes and reduced colloid. The presence of congenital goiter in both the proband and sibling with fetal goiter raised suspicion of TDH. Serum TG levels were markedly low (0.2 ng/mL;range: 3.5–77 ng/mL). Targeted next-generation sequencing identified a novel homozygous TG variant (NM_003235): c.5686+3A>C/p.(?) Parents were heterozygous. This variant was classified as a variant of uncertain significance (VUS) (PP3, Pm²) according to ACMG criteria. In silico prediction tools defined a splice-altering effect, with strong pathogenicity scores. The variant was absent from gnomAD. A prenatal ultrasound in the mother’s new pregnancy detected fetal goiter, prompting in-utero L-thyroxine. Segregation analysis confirmed the homozygous variant. To assess its splicing impact, RNA from peripheral blood (proband and affected sibling) was reverse-transcribed into cDNA, amplified by PCR, and sequenced. The splicing analysis confirmed aberrant exon skipping, specifically predicted in-frame skipping of exon 30 leading to c.5549_5686del/p.(Leu1851Cys (;)1852_1896del) with loss of 45 residues from type IIIA and type IIIB repeat regions (UniProt).

Conclusion: This study identifies a novel TG splice-site variant, expanding the genetic landscape of CH. Functional RNA analysis confirmed its pathogenicity by demonstrating aberrant splicing. Our findings underscore the importance of early genetic diagnosis in dyshormonogenesis, enabling timely genetic counseling.