1. Clinical classification of thyroiditis:
3) Chronic (most frequent).
2. Histologic classification of thyroiditis:
1) Bacterial thyroiditis (acute suppurative thyroiditis).
2) Other (nonbacterial) acute thyroiditis: Radiation-induced thyroiditis; palpation-induced or trauma-induced thyroiditis; drug-induced thyroiditis (interferon alpha, interleukin 2, amiodarone, or lithium).
3) Subacute granulomatous thyroiditis (synonyms: subacute thyroiditis, granulocytic thyroiditis, giant cell thyroiditis, de Quervain thyroiditis).
4) Subacute lymphocytic thyroiditis (painless thyroiditis; postpartum thyroiditis).
5) Chronic lymphocytic thyroiditis (also called chronic autoimmune thyroiditis and Hashimoto thyroiditis; most common).
6) Chronic infiltrative fibrosing thyroiditis (Riedel thyroiditis).
3. Classification of thyroiditis based on thyroid function:
1) Destructive thyroiditis with periodic or continuous thyrotoxicosis caused by the destruction of thyroid follicles and release of variable (sometimes very high) amounts of thyroid hormones to the circulation. This class includes many types of thyroiditis with different etiologies; most frequently, they are acute or subacute, but chronic thyroiditis may also occur. Pain or tenderness are not always present and are not required for the diagnosis. The clinical spectrum includes numerous types of thyroiditis:
a) Subacute painful granulomatous thyroiditis (de Quervain thyroiditis).
b) Viral painless granulomatous thyroiditis.
c) Subacute painless lymphocytic thyroiditis (silent thyroiditis).
d) Painful lymphocytic thyroiditis.
e) Posttraumatic thyroiditis (including palpation-induced thyroiditis).
f) Radiation-induced thyroiditis.
g) Infectious thyroiditis.
h) Drug-induced thyroiditis, for instance, caused by amiodarone, interferon alpha, interleukin 2, or lithium, and recently tyrosine kinase inhibitors.
2) Nondestructive thyroiditis: All the other conditions that do not cause thyrotoxicosis.