We describe a case of subclinical hyperthyroidism in a 77 years-old patient. This presentation, even if not clinically extreme, could lead to a decrease in the quality of life, but also to cardiac arrhythmia. This could also initiate more serious complications. We describe in the clinical story some of the signs that should draw suspicion of subacute hyperthyroidism. We recall factors that could potentially delay the diagnosis. Finally, we describe the use of different biological tests, ultrasound imaging and nuclear medicine.