Challenging diagnosis of thyroid hormone resistance initially as Hashimoto's thyroiditis

Background Resistance to thyroid hormone (RTH) commonly presents with goiter, attention deficit hyperactivity disorder (ADHD), short stature and tachycardia. However, due to its variable presentation with subtle clinical features, a third of the cases are mistreated, typically as hyperthyroidism. Case presentation A 15-year-old female with ADHD and oligomenorrhea was initially diagnosed as Hashimoto's thyroiditis but found to have a rare heterozygous mutation in c803 C>G (p Ala 268 Gly) in the THRβ gene, confirming resistance to thyroid hormone. Conclusions Fluctuating thyroid function tests in addition to thyroid peroxidase antibody (TPO Ab) positivity complicated the diagnosis of RTH, initially diagnosed as Hashimoto's thyroiditis. A high index of suspicion is needed to prevent misdiagnosis and mistreatment.