Alpha-fetoprotein (AFP) is a biomarker commonly used in the diagnosis of various malignancies but may also be elevated in non-neoplastic conditions, including hypothyroidism. We report the case of a 3-year-old girl with Down syndrome (DS) and newly diagnosed hypothyroidism, who presented with a hypoechoic oval lesion adjacent to the thymic parenchyma on ultrasound and markedly elevated AFP levels (169.2 ng/mL). Further investigations, including MRI, excluded the presence of germ cell tumors. Following initiation of levothyroxine therapy, AFP levels normalized in parallel with thyroid function. No evidence of malignancy was detected despite the initial suspicion. This case underscores the association between elevated AFP and hypothyroidism, highlighting the importance of evaluating thyroid status in patients with increased AFP to avoid unnecessary oncological investigations. In particular, elevated AFP in the context of hypothyroidism and DS warrants careful thyroid assessment and follow-up to prevent redundant diagnostic procedures and reduce patient and family anxiety. Thyroid function testing should be considered before extensive oncological evaluation in children with elevated AFP.
