**2. Hypopituitarism**

Hypopituitarism as an irAE is more common in patients receiving anti-CTLA-4 antibodies than in those receiving anti-PD-1/anti-PD-L1 antibodies, with the reported incidences being approximately 10% and ≤1% [17–22]. In addition, it has been reported that the incidence of hypophituitarism is higher with the concomitant use of anti-CTLA-4, and anti-PD-1/PD-L1 antibodies are more common than the use of a single agent [12]. Hypopituitarism caused by ICI treatment is classified into hypophysitis and isolated adrenocorticotropic (ACTH) deficiency. Pituitary gland enlargement is seen in hypophysitis, which causes hyposecretion of several anterior pituitary hormones, including thyroid-stimulating hormone (TSH), gonadotropins, and ACTH. On the other hand, the pituitary gland does not enlarge in ACTH deficiency, wherein the secretory capacity of only ACTH is reduced. There are very few reports of posterior pituitary dysfunction [17,23]. Although both patterns of dysfunction can occur in patients receiving anti-CTLA-4 antibodies, the use of anti-PD-1/anti-PD-L1 antibodies has been associated with ACTH deficiency in most cases [17,20,24]. Hypopituitarism often develops 4–10 weeks after treatment initiation due to anti-CTLA-4 antibodies [12]. An association between the incidence and the dose has also been noted, with one report showing a two-fold higher incidence in patients receiving high-dose ipilimumab (10 mg/kg) than in those receiving low-dose ipilimumab (3 mg/kg) [19]. Moreover, the higher dose (10 mg/kg) resulted in more adverse events than did the lower dose (3 mg/kg). However, significantly longer survival associated with the higher dose has been documented in some reports, and an association between irAE development and treatment efficacy has been pointed out [18,24]. Hypopituitarism also occurs within months to 1 year after treatment initiation due to anti-PD-1/anti-PD-L1 antibodies, and it may even develop after discontinuation of the drug [25–27]. It should be noted that ACTH hyposecretion always develops in all cases of hypopituitarism due to ICI treatment. The symptoms of ICI-induced hypopituitarism include anorexia and malaise due to secondary adrenal insufficiency, weight loss, gastrointestinal symptoms (nausea, vomiting, and diarrhea), hypotension, and hypoglycemia. In addition, headache, visual field impairment, and visual impairment occur in cases of hypophysitis with high-grade enlargement of the pituitary gland. In blood examination, abnormal findings such as hyponatremia and eosinophilia are recognized.

If hypopituitarism is suspected, it is necessary to measure the hormones secreted by the anterior pituitary gland and target organs. With regard to hypophysitis, diffuse enlargement and swelling of the pituitary gland and pituitary stalk with enhancement on contrast-enhanced magnetic resonance imaging are observed in more than half of the cases [17]. Subsequently, the enlarged pituitary gland gradually shrinks in the acute phase, and pituitary function is partially or completely lost [28,29]. During long-term observations, (median follow-up, 33 months) in one study, many of the thyroid and gonadal dysfunctions were found to be reversible, whereas ACTH hyposecretion was irreversible in most cases [29]. The pathogenesis of hypopituitarism due to ICI treatment remains unclear. In autopsied cases of pituitary dysfunction caused by tremelimumab, anti-CTLA-4 antibody, necrotic changes, and lymphocytic infiltrates with fibrosis were observed in the anterior pituitary gland. In addition, complement component 4 fragment (C4d) deposition associated with complement activation was observed; this suggested the involvement of both type IV and type II allergic reactions [17].
