Factitious thyrotoxicosis (FTT) is a common form of thyroid hormone (TH) abuse involving voluntary but concealed intake of an excessive amount of TH. In most cases, FTT seeks to improve body composition with a decrease in body fat and weight while maintaining apparent fitness. It is frequent in Munchausen syndrome, to attract attention for care. It can involve excessive intake either of thyroxine (T4) or of thyroid extracts or liothyronine (T3). In addition, several dietary supplements available on-line were shown to contain clinically relevant amounts of T4 and T3. TH abuse also occurs in elite athletes and bodybuilders, to reach the appropriate weight and prioritize fat loss. Diagnosis should be suspected whenever the typical features of hyperthyroidism or endogenous thyrotoxicosis are not present, as prolonged overlooked TH abuse can lead to severe consequences, including life-threatening events.