A three-year-old girl with a history of recurrent tonsillitis was investigated for failure to thrive and global developmental delay. Clinically, she had a triangular face with low-set ears and intermittent tachycardia. She had growth failure with her weight under the third centile while her height was within normal limits. Other systemic examinations were unremarkable. The presence of an elevated free T4 (FT4) with an inappropriately high thyroid stimulating hormone (TSH) in this patient raised the clinical suspicion of Thyroid Hormone Resistance Syndrome. DNA sequencing confirmed the diagnosis, which showed R243W gene mutation in Thyroid Hormone Receptor-Beta1 (THRB1).