Abstract

Resistance to thyroid hormone beta (RTHβ) is a rare condition typically caused by mutations in the THRβ gene, characterized by elevated thyroid hormones with non-suppressed TSH levels. We present a pediatric case of RTHβ associated with a novel THRβ variant, emphasizing diagnostic challenges and the importance of individualized treatment. A 6.5-year-old girl was evaluated for learning difficulties and tachycardia. Laboratory findings showed elevated free T3 and T4 with non-suppressed TSH. Pituitary MRI showed a 5x6 mm lesion, raising suspicion for TSHoma. Genetic testing of the THRβ gene was performed. A novel heterozygous THRβ variant (c.1376T>C; p.Phe459Ser) was identified in both the patient and her father. The mutation affects a highly conserved residue within the ligand-binding domain. Clinical and biochemical findings were consistent with RTHβ. Atenolol therapy was initiated to manage tachycardia with favorable response.This case highlights the potential for misdiagnosis of RTHβ as TSHoma and underscores the value of genetic testing in differentiating the two. The identification of a novel variant at codon 459 expands the mutational spectrum of THRβ and supports its role as a hotspot region relevant to RTHβ pathogenesis.