Immune checkpoint inhibitor-induced diabetes

Immune checkpoint inhibitor-induced diabetes is a specific form of iatrogenic diabetes that develops during treatment with anticancer immunotherapies capable of removing the main physiological restraints on T-lymphocyte responses, particularly through blockade of programmed cell death protein 1 (PD-1), programmed death ligand 1 (PD-L1), or cytotoxic T-lymphocyte-associated protein 4 (CTLA-4). Unlike other forms of drug-induced diabetes, in which insulin resistance or indirect metabolic toxicity is the dominant mechanism, this condition is characterized primarily by rapid autoimmune injury to pancreatic beta cells, with consequent loss of insulin secretion and a high risk of acute presentation with diabetic ketoacidosis. In the most recent literature, this entity is often termed immune checkpoint inhibitor-associated diabetes mellitus or checkpoint inhibitor-associated autoimmune diabetes mellitus, specifically to emphasize its pathogenic relationship with treatment-induced immune dysregulation.

Its clinical relevance is very high despite its relative rarity, because it may develop abruptly in patients with cancer and no previous history of diabetes, may present as a metabolic emergency, and often results in persistent insulin deficiency. Moreover, the problem concerns not only endocrinological management of glucose but also coordination with the oncology team, because the endocrine adverse event occurs within the broader spectrum of immune-related toxicities caused by immunotherapy. The most recent updates to the American Diabetes Association standards added a specific recommendation on plasma glucose monitoring in people treated with immune checkpoint inhibitors, confirming that this form is no longer regarded as a descriptive exception but as a recognized clinical condition requiring dedicated surveillance.

Epidemiologically, immune checkpoint inhibitor-induced diabetes is considered a rare complication, but its absolute frequency has increased in parallel with the expansion of immunotherapy across numerous solid and hematologic malignancies. Recent reviews indicate that the incidence reported in published cohorts is generally below 2%, often ranging approximately from 0.2% to 1.9%, although these estimates depend heavily on the type of drug, treatment combination, population studied, and definition used to identify cases. The event appears to be observed more often with agents targeting PD-1 or PD-L1 than with isolated CTLA-4 blockade, although combination regimens may increase the overall risk of immune-related endocrine toxicity.

The clinical importance of this form greatly exceeds its numerical frequency because onset is often abrupt and severe. In many case series, a substantial proportion of patients are already in diabetic ketoacidosis when first assessed, clearly distinguishing this entity from type 2 diabetes mellitus or other more gradual iatrogenic forms. This acute presentation reflects the fact that functional beta-cell mass may decline rapidly over a relatively short period, leaving little room for a prolonged, recognizable prodromal phase. Its immediate prognostic impact therefore depends less on overall prevalence than on the severity of its mode of onset.

The timing of onset in relation to immunotherapy initiation is variable. Reported cases may develop after only a few administrations or after several months of treatment, and there is no single, universally predictable time window. The literature stresses precisely this heterogeneity, which makes it impossible to rely on surveillance limited to the first few weeks. Although risk may be higher during the early stages of exposure in some series, it persists throughout treatment and may occasionally become apparent even after relatively recent treatment discontinuation.

Epidemiologically, it must also be recognized that checkpoint inhibitor-induced diabetes belongs to the broader group of immune-related endocrinopathies, alongside thyroid disorders, hypophysitis, adrenal insufficiency, and other endocrine dysfunctions. This has two practical implications. First, a patient may develop more than one endocrinopathy during the same course of treatment. Second, the appearance of one endocrine toxicity increases clinical vigilance for other manifestations within the same spectrum. Immune checkpoint inhibitor-induced diabetes should therefore not be evaluated in isolation, but within a syndrome of immune-mediated endocrine toxicity caused by anticancer therapy.

The pathogenesis of this form of diabetes differs profoundly from that of glucocorticoid-induced diabetes, antipsychotic-associated diabetes, or diabetes caused by other metabolic drugs. The dominant mechanism is not a primary increase in insulin resistance, but a breakdown of immune tolerance toward the pancreatic beta cell. The PD-1, PD-L1, and CTLA-4 immune checkpoints normally restrain immune responses by limiting the activation and persistence of autoreactive T lymphocytes. Their pharmacological blockade, which is useful in enhancing antitumor immunity, may also remove physiological barriers to self-tolerance and permit an immune attack on the endocrine pancreas in susceptible individuals.

Available pathogenic evidence indicates that beta-cell injury is often rapid and substantial. Clinical and pathological studies cited in recent reviews have documented markedly low C-peptide, lymphocytic infiltration of the pancreatic islets in selected cases, and, in some series, an association with pancreatic atrophy. This profile suggests more rapid beta-cell destruction than that observed in slowly progressive classic autoimmune diabetes of adulthood. The extent of the secretory deficit explains why many patients present with severe hyperglycemia and ketosis despite a glycated hemoglobin level that is not necessarily very high, indicating a process that developed over a relatively short period.

The immunogenetic profile appears to play an important but not exclusive role. Some series have found an association with human leukocyte antigen (HLA) haplotypes typically linked to autoimmune diabetes, whereas others show that the immunogenetic pattern does not completely overlap with that of classic type 1 diabetes mellitus. Autoimmune serology is also heterogeneous. Some patients have autoantibodies associated with autoimmune diabetes, such as antibodies against glutamic acid decarboxylase (GAD), whereas in other cases serology is negative despite the insulin-deficient clinical phenotype. The absence of autoantibodies therefore does not exclude the diagnosis, and checkpoint inhibitor-induced diabetes cannot be reduced to a simple replica of spontaneous type 1 diabetes.

The final pathophysiological result is an abrupt decline in insulin secretory capacity, with loss of lipolysis suppression, increased hepatic ketogenesis, and rapid progression toward absolute or near-absolute insulin deficiency. This state explains the high frequency of diabetic ketoacidosis and clarifies why glucocorticoids, which are useful for other immune-related toxicities, do not restore beta-cell function once the condition is established. The resulting clinical logic is that, metabolically, this form behaves much more like acute insulin-deficient diabetes than like simple, reversible drug-induced hyperglycemia.

Clinical manifestations should be assessed in a sequence resembling an actual medical evaluation. In the initial history, patients may report the classic symptoms of hyperglycemia, namely polyuria, polydipsia, weight loss, severe fatigue, dry mouth, blurred vision, and worsening general well-being. In several cases, however, the prodromal course is very brief and the first medical contact occurs only after metabolic decompensation has developed, with nausea, vomiting, abdominal pain, tachypnea, drowsiness, or confusion consistent with diabetic ketoacidosis. The frequency of presentation with ketoacidosis is one of the most distinctive features of this entity compared with other forms of iatrogenic diabetes.

An important clinical issue is that patients with cancer receiving immunotherapy may attribute their initial symptoms to other causes, such as the malignancy itself, treatment-related fatigue, gastrointestinal symptoms, or nonspecific systemic toxicity. This makes the condition potentially insidious. Intense thirst or polyuria may initially be underestimated; nausea and vomiting may be interpreted as oncological adverse effects; weight loss may be attributed to the underlying disease. Clinicians should therefore maintain a low diagnostic threshold, especially when nonspecific symptoms are accompanied by rapid clinical deterioration.

Physical findings may range from mild dehydration to a clear metabolic emergency. Tachycardia, relative hypotension, mucocutaneous dehydration, a ketotic odor, deep Kussmaul breathing, reduced consciousness, or signs of hypoperfusion may be present. In other patients, objective findings are less striking and recognition arises primarily from the treatment context and laboratory data. In all cases, the correct clinical sequence requires moving from reported symptoms to estimation of acute risk, because delayed diagnosis can transform a manageable endocrine toxicity into an event requiring intensive care.

It should also be remembered that checkpoint inhibitor-induced diabetes may coexist with other immune-related endocrinopathies, particularly thyroid disorders and, less frequently, hypophysitis or adrenal insufficiency. The clinical picture may therefore be mixed and include overlapping symptoms such as fatigue, weight loss, gastrointestinal disturbances, or blood pressure changes. Clinicians must avoid a single-cause interpretation of symptoms and always consider the possibility of multiple endocrine toxicities in the same patient.

Clinical suspicion should arise in any patient receiving an immune checkpoint inhibitor who develops new-onset hyperglycemia, rapid worsening of previously stable diabetes, or symptoms consistent with acute insulin deficiency. The temporal relationship with immunotherapy is essential, but is not sufficient on its own. The specific agent used, presence of other immune-related toxicities, speed of symptom onset, and above all the risk of ketoacidosis must be considered. Suspicion should be immediate when a patient reports polyuria, polydipsia, nausea, vomiting, or marked fatigue during immunotherapy.

The medication history must be accurate and specific. It is not enough to know that the patient “is receiving immunotherapy.” Clinicians must determine whether the patient is receiving an anti-PD-1, anti-PD-L1, anti-CTLA-4 agent, or a combination; the line of treatment; the administration schedule; and any concomitant medications, particularly glucocorticoids or other agents that may cause hyperglycemia through different mechanisms. This chronological reconstruction is essential to distinguish acute autoimmune diabetes from stress hyperglycemia, steroid-induced hyperglycemia, or progression of pre-existing type 2 diabetes mellitus.

Features that strongly support suspicion of immune checkpoint inhibitor-induced diabetes

• Rapid onset of hyperglycemia during treatment with anti-PD-1, anti-PD-L1, or combination immunotherapy
• Association with ketonemia, diabetic ketoacidosis, or symptoms consistent with severe insulin deficiency
• Low or rapidly declining C-peptide, even when glycated hemoglobin is not markedly elevated
• Concurrent or previous presence of other immune-related endocrinopathies

The threshold for concern must also be very low in patients with known diabetes. A sudden rise in glucose values, especially when accompanied by ketosis or a rapid decline in C-peptide, should not automatically be attributed to oncological stress or poor treatment adherence. In this setting, the key feature prompting suspicion is not simply hyperglycemia, but a change in metabolic phenotype toward a more insulin-deficient and unstable state.

Diagnosis is constructed in two logical stages. First, the presence of diabetes or severe hyperglycemia must be confirmed according to general criteria. Under the American Diabetes Association's Standards of Care in Diabetes 2026, diabetes may be diagnosed using fasting plasma glucose, 2-hour plasma glucose after an oral glucose tolerance test (OGTT), glycated hemoglobin (HbA1c), or random plasma glucose in the presence of classic symptoms of hyperglycemia or a hyperglycemic crisis. The second stage defines the nature of the toxicity by showing that this is probably a form induced by immune checkpoint inhibitors rather than another cause of glycemic decompensation.

Initial investigations must be guided first by clinical severity. In the presence of marked hyperglycemia, gastrointestinal symptoms, dehydration, or altered consciousness, blood or urine ketones, electrolytes, kidney function, osmolality when indicated, and acid-base balance are required, because the immediate priority is to exclude or confirm diabetic ketoacidosis. At the same time, secretory reserve should be assessed with C-peptide, ideally interpreted in the context of the concurrent glucose value, because a low or inappropriately normal value in the presence of marked hyperglycemia suggests substantial insulin deficiency.

Testing for autoantibodies associated with autoimmune diabetes, such as anti-GAD and other beta-cell markers, may provide useful information but is not essential for the clinical diagnosis, because a substantial proportion of patients are seronegative. HbA1c must be interpreted carefully: values that are not markedly elevated do not exclude severe onset and may actually strengthen the hypothesis of recent beta-cell destruction when accompanied by marked hyperglycemia and low C-peptide. In the literature, this combination—very high glucose, a secretory deficit, and only moderately increased HbA1c—is regarded as a typical pattern of rapidly developing forms.

In the absence of a single set of internationally accepted official diagnostic criteria for this specific toxicity, diagnosis is essentially clinical and pathogenetic. According to available reviews and practice documents, immune checkpoint inhibitor-induced diabetes requires documentation of new diabetes or abrupt glycemic deterioration in a patient exposed to checkpoint immunotherapy, together with evidence consistent with rapid insulin deficiency, such as ketosis, ketoacidosis, or low C-peptide, after more convincing alternative explanations have been excluded. The core of diagnosis is therefore not a single pathognomonic test, but the convergence of treatment chronology, metabolic phenotype, and markers of secretory loss.

The differential diagnosis primarily includes previously unrecognized type 2 diabetes mellitus, glucocorticoid-induced diabetes, stress hyperglycemia, immune-related pancreatitis with endocrine involvement, artificial nutrition, and decompensation of pre-existing nonautoimmune diabetes. The distinction may be complex in some patients, but the combination of rapid onset, a tendency toward ketoacidosis, and a severe reduction in insulin secretion strongly supports checkpoint inhibitor-associated diabetes.

Nosologically, this condition is included among other specific types of diabetes, and more precisely among drug-induced forms, but stopping at this general label would be reductive. Clinically, immune checkpoint inhibitor-induced diabetes resembles rapidly developing, insulin-deficient autoimmune diabetes far more than a simple iatrogenic glycemic disorder caused by insulin resistance. For this reason, much recent literature uses terms that emphasize its autoimmune nature, such as checkpoint inhibitor-associated autoimmune diabetes mellitus.

A useful practical classification distinguishes cases with fulminant or near-fulminant onset, often accompanied by ketoacidosis and very low C-peptide, from cases diagnosed at an earlier stage before severe decompensation. This distinction does not change the underlying pathogenesis, but helps identify the point along the clinical trajectory at which the patient was detected. Another useful distinction concerns the presence or absence of autoantibodies and any coexistence of other immune-related endocrinopathies, which help define the individual immunological profile without changing the need for insulin therapy when the secretory deficit is marked.

It should also be made clear that not every case of hyperglycemia during immunotherapy belongs to this category. Some patients have multifactorial hyperglycemia related to glucocorticoids, inflammatory stress, infections, nutrition, or worsening pre-existing type 2 diabetes. Correct classification therefore requires distinction between immunotherapy-associated hyperglycemia and true immune checkpoint inhibitor-induced autoimmune diabetes, which entails a significant loss of beta-cell function. This distinction is crucial because it has direct implications for treatment, follow-up, and the likelihood of permanent insulin dependence.

Treatment is based on an essential principle: when the condition is genuinely immune checkpoint inhibitor-induced diabetes with severe loss of beta-cell function, the cornerstone of therapy is insulin. Unlike other immune-related endocrinopathies, in which glucocorticoids may modulate inflammation and promote functional recovery, there is no evidence here that corticosteroids reliably restore insulin secretion once beta-cell injury has become clinically manifest. Metabolic management must therefore follow the logic of insulin-deficient diabetes rather than that of simple reactive hyperglycemia.

If the patient presents with diabetic ketoacidosis, treatment must be immediate and follow standard protocols for this acute complication, with volume expansion, insulin infusion, electrolyte correction, close monitoring, and investigation of concurrent aggravating factors. The priority at this stage is metabolic stabilization rather than precise final subtype classification. Only after the emergency has been controlled should the etiological assessment be completed and chronic treatment planned.

Once the acute phase has resolved, most patients require a basal-bolus insulin regimen or otherwise intensive insulin replacement appropriate to the degree of insulin deficiency. Patient education must be comprehensive and include self-monitoring, prevention of hypoglycemia, sick-day management, recognition of ketosis, and, where appropriate, principles of carbohydrate counting. Recent literature emphasizes that insulin dependence persists over time in many cases because recovery of endogenous secretion is limited or absent.

The decision whether to continue immunotherapy cannot be made automatically by the endocrinologist alone. It must be shared with the oncology team and depends on event severity, tumor response, treatment alternatives, and the patient's overall condition. Available sources indicate that, once properly treated, diabetes does not always and necessarily require permanent discontinuation of the checkpoint inhibitor, but it does require individualized assessment. In other words, diabetes should not be viewed solely as metabolic toxicity, but as a factor that must be balanced against the expected oncological benefit.

Non-insulin glucose-lowering medications may have only a marginal role in selected situations and are not the cornerstone of management when the principal defect is loss of insulin secretion. The correct therapeutic rationale is therefore simple but stringent: identify insulin deficiency rapidly, treat it with replacement therapy, and integrate diabetes management into the overall cancer care pathway.

Follow-up must begin during cancer treatment itself. The 2026 updates to the ADA standards recommend plasma glucose monitoring at every visit in people treated with immune checkpoint inhibitors, specifically to promote early recognition of this complication. This recommendation is important because symptom-based surveillance alone may be insufficient: the patient may present late, and the interval between the first signs of hyperglycemia and ketoacidosis may be brief.

After diagnosis, follow-up should be structured as for a high-risk insulin-deficient form of diabetes. Frequent glucose checks, periodic reassessment of insulin requirements, monitoring for hypoglycemia, ongoing education, and attention to settings that increase the risk of decompensation—such as infections, reduced oral intake, vomiting, or other oncological toxicities—are required. In patients using continuous glucose monitoring (CGM), glycemic variability can be followed in greater detail, although the specific literature in this population is still evolving.

Another crucial aspect of follow-up is surveillance for other immune-related endocrinopathies. A patient with checkpoint inhibitor-induced diabetes should not be regarded as having an isolated toxicity, but as an individual who has already demonstrated susceptibility to immunotherapy-induced autoimmune endocrine events. It is therefore rational to maintain clinical vigilance regarding thyroid function, the adrenal axis, and other endocrine organs according to the clinical context and the oncology protocols in use.

Over the medium to long term, follow-up should determine whether there is any partial recovery of insulin secretion. Current evidence suggests that this occurs rarely and that many patients remain insulin-dependent. Periodic reassessment of C-peptide and glycemic control may nevertheless help define the individual's evolving phenotype and guide treatment adjustments. In all cases, follow-up must never assume automatic reversibility of the condition.

Immediate prognosis depends mainly on how rapidly onset is recognized and any diabetic ketoacidosis is treated. The principal acute risk is the metabolic crisis itself, which may be severe and require admission to an intensive or intermediate care unit. Timely diagnosis significantly reduces this risk and permits more rapid clinical stabilization. Long-term metabolic prognosis, by contrast, is determined primarily by the degree of residual beta-cell loss. Recent reviews agree that insulin deficiency persists in a very large proportion of patients and requires long-term replacement therapy.

Acute complications include diabetic ketoacidosis, severe dehydration, electrolyte disturbances, and recurrent glycemic decompensation, particularly when the initial diagnosis is delayed or therapeutic education is incomplete. Over the long term, if metabolic control remains inadequate, the patient enters the same continuum of risk as other forms of insulin-deficient diabetes, with potential development of microvascular and macrovascular complications. Follow-up duration in published cohorts is still relatively limited compared with other forms of diabetes, but the biological principles of chronic hyperglycemic toxicity remain valid in this population.

There is also an oncological prognostic dimension. Immune checkpoint inhibitor-induced diabetes may complicate continuation of antitumor treatment, require temporary interruptions, or necessitate multidisciplinary reassessment of the risk-benefit balance. However, the onset of diabetes alone does not automatically mean failure of the cancer treatment pathway. When metabolic stabilization is effective and the patient is followed appropriately, the antitumor strategy can often be continued or resumed according to specialist judgment. Overall prognosis therefore depends on a dynamic balance among cancer control, endocrine-metabolic stability, and the care team's ability to integrate these two objectives.

In summary, this form of diabetes is rare but has a high clinical impact. Prognosis improves the earlier the etiological relationship with immunotherapy is recognized, insulin deficiency is identified, and appropriate replacement therapy is started without delay. The decisive issue is not merely controlling glucose, but promptly detecting an acute autoimmune transition toward insulin deficiency in a patient who is already clinically complex because of cancer and immunological treatment.