The case of an association of three well-differentiated pathologies is reported. A 40-year-old woman with a toxic thyroid adenoma developed acute polyarthritis during the course of the disease. The articular symptoms together with chest X-rays and a consistently negative PPD (1:100) were suggestive of sarcoidosis. This was later confirmed by the discovery of non-caseating granulomas in the liver and superficial lymph nodes, and by a positive Kveim's test. Histopathologic examination of the thyroid gland confirmed the existence of the adenoma. Sarcoid infiltration was not observed, but on the other hand the typical images of subacute granulomatous thyroiditis (De Quervain's thyroiditis) were discovered.