The corticosteroid-responsive encephalopathy associated with autoimmune thyroiditis (the so-called "Hashimoto's Encephalopathy") is a rare disorder with multiple symptomatology, breaking out with an acute or subacute onset and having a relapsing course, not correlated to thyroid hormone levels, with autoimmune pathogenesis, and usually associated with Hashimoto's thyroiditis. In this paper, we report on a case study regarding a 46 year-old woman showing a subacute course cerebellar syndrome, associated with Hashimoto's thyroiditis, diagnosed as "Hashimoto's encephalopathy". The possible pathogenesis and the major aspects of the differential diagnostic sector are discussed with particular reference to an ataxic syndrome caused by a progressive non-familial adult onset cerebellar degeneration (PNACD), associated with the thyroid disease itself.