Subacute granulomatous thyroiditis

1. Acute - Bacterial abscess- rare.
2. Subacute granulomatous (de Quervain's) thyroiditis. Uncommon. A viral etiology is suspected but not proved. The patient has symmetrical or focal enlargement with a painful and firm gland (hence also known as 'painful thyroiditis'). Transient thyrotoxicosis may be present due to destruction of the thyroid follicles with release of thyroid hormones. Microscopically, the gland shows granulomatous inflammation, histiocytic giant cells, and lymphocytes. It is a self-limiting disease and the patient generally recover after several weeks or months.
3. Subacute lymphocytic thyroiditis: Also referred to as "painless", "silent", or "postpartum" thyroiditis. Histologically, it is part of the spectrum of autoimmune thyroditis and the biopsy shows lymphocytic infiltrate but not as striking as in Hashimoto's thyroiditis. Moreover, the autoimmune reactions are inconstant and evanescent. Clinically, it is regarded as a form of "subacute thyroiditis". Fifty percent of these patients will develop transient thyrotoxic symptoms because an excess of preformed hormone leaks from the thyroid gland due to inflammation. New hormone formation is decreased however, owing to the suppression of TSH secretion by the hormone excess. Since the inflammatory disorder is transitory and since stores of preformed hormones are ultimately depleted, the thyrotoxicosis is self-limited and often followed by a transient period of thyroid hormone insufficiency (hypothyroidism). However, a small number of the patients assessed a decade later will show permanent hypothyroidism due to more severe inflammation destroying the thyroid parenchyma.
4. Chronic lymphocytic (Hashimoto's) thyroiditis. This is the most common form of thyroiditis. It is an autoimmune disease. Classically, the patient has painless bilateral diffuse enlargement caused by lymphoid infiltration. The serum antimicrosomal antibody titre is elevated. The thyroid follicles undergo destruction and the cytoplasm of the follicular cells undergo eosinophilic change (Hurthle cells). The inflammation may resolve; if not, fibrosis may develop. Patients are euthyroid until the late (fibrotic) stage; then often become hypothyroid (NOTE: Hashimoto's thyroiditis is a frequent cause of hypothyroidism).
5. Riedel's Thyroiditis - Very rare disease characterized by dense fibrosis (stony hard gland). It is often associated with idiopathic fibrosis of the mediastinum and retroperitoneum.

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