BACKGROUND: Thyroid hormones are crucial for metabolism in all tissues in humans, including the nervous system and muscles, and could thus affect handwriting, which is the synthesis of complex and fine movements. Hyperthyroidism, characterized by symptoms such as tremor and weakness, could affect handwriting, although this has not been studied yet. The aim of this study was to evaluate handwriting characteristics before and after therapy for hyperthyroid Graves' disease (GD). METHODS: Twenty-two patients (15 women, 7 men) with untreated GD (median age: 44 years; range: 20-70 years) were asked to write a "standard text" before and 12 months after being rendered euthyroid. The letters underwent a standardized detailed analysis by a handwriting expert, through inspection and stereoscopic microscope and magnifying glass. RESULTS: All patients demonstrated handwriting variations, perceptible even to direct observation. Graphological examination showed statistically significant changes after patients become euthyroid, in the following parameters: size of letters (4.5±1.1 vs. 5.9±1.3 mm; p<0.01), distance between letters (62.9±1.1 vs. 55.2±0.8 mm; p<0.01), width of letters (1.75±0.06 vs. 2.2±0 .06 mm; p<0.01), distance between words (216.2±3.2 vs. 198.7±2.4 mm; p<0.01), extension of letters (8.7±0.2 vs. 7.7±0.2 mm; p<0.01), angles (17±0.3 vs. 15.8±0.4 mm; p<0.01), and groove depth (0.2±0.05 vs. 0.4±0.05 mm; p<0.01). CONCLUSIONS: Hyperthyroid GD was associated with significant changes in handwriting in all patients. Following recovery from hyperthyroidism, a state of hypertrophic and contracted handwriting resulted in greater fluency and fluidity. Variations in handwriting should be included as signs/symptoms in GD.