What are the main types of primary hypophysitis and their characteristics?
Primary hypophysitis affects an estimated 1.9 million people in the United States and is considered an autoimmune disorder. It is subclassified by the primary inflammatory cell sub-type and includes the following main causes and characteristics:
1. Lymphocytic hypophysitis: The most common form of hypophysitis and is characterized by lymphocytic infiltrates of the pituitary gland and/or stalk. It was initially thought to occur almost exclusively in young females during the peripartum period, but additional studies showed that this entity also affects males and comprises ≈ 30% of all hypophysitis cases. Preferential deficiencies of adrenocorticotropic hormone (ACTH)– and thyroid-stimulating hormone (TSH)–secreting cells have been described in many, but not all, reports.
2. Plasma cell (immunoglobulin G4-related disease [IgG4-RD]) hypophysitis: A newly-described entity since 2011, and a common cause of primary hypophysitis (≈ 30%). Histologically, it is characterized by infiltration with plasma cells, which produce IgG4, although IgG4 antibodies are not considered pathogenic by themselves. Infiltration of the pituitary is often associated with infiltration of other organs, such as the pancreas (autoimmune pancreatitis), thyroid (Hashimoto’s and Riedel’s thyroiditis), lungs (interstitial pneumonia). It can also cause retroperitoneal fibrosis, sclerosing cholangitis, and retroorbital pseudotumor, among other disorders.
3. Granulomatous hypophysitis: The third most common type of primary hypophysitis. This condition is characterized histologically by infiltration with histiocytes and giant cells.
4. Xanthomatous hypophysitis: Generally considered the rarest type of primary hypophysitis. It is characterized by the presence of xanthomatous cells (macrophages) and may be associated with progression to xanthogranulomas.
5. Mixed forms of hypophysitis: Lymphogranulomatous hypophysitis and xanthogranulomatous hypophysitis are mixed forms of hypophysitis.