Chronic invasive fibrous thyroiditis (Riedel's struma) is a very rare disease of 
unknown aetiology mainly affecting middle-aged or old female patients. An 
aggressive fibrosis partly or totally replaces normal thyroid gland tissue. The 
gland becomes stony hard, is not displaceable and, characteristically, the 
fibrous tissue penetrates the capsule and infiltrates into surrounding 
structures such as muscles, vessels, nerves and even the trachea. Riedel's 
struma is often associated with fibrotic processes in other parts of the body. 
The preoperative differential diagnosis includes malignant tumours and fibrosing 
stages of Hashimoto's disease, as well as subacute thyroiditis de Quervain. This 
paper reports the case of a 60-year-old woman suffering from Riedel's struma and 
discusses differential diagnostic aspects with regard to preoperative 
investigation and pathohistology.