Diabetes develops when your body either stops making insulin, stops responding to it properly, or both. The specific cause depends on the type of diabetes, and there are more types than most people realize. Type 2 accounts for roughly 90-95% of all cases, type 1 makes up about 5-10%, and rarer forms like gestational diabetes, LADA, and monogenic diabetes fill in the rest. Each has a distinct cause, though they all end at the same place: blood sugar that stays too high.
Type 1: The Immune System Attacks
Type 1 diabetes is an autoimmune disease. Your immune system, which normally fights infections, mistakenly targets and destroys the insulin-producing beta cells in your pancreas. The attack is carried out primarily by T cells, a type of white blood cell. Two varieties do most of the damage: CD4 T cells coordinate the assault by recruiting other immune cells, while CD8 T cells directly kill beta cells through contact, puncturing them with specialized proteins called perforin and granzyme.
The destruction feeds on itself. Immune cells in the pancreas release inflammatory signals that activate macrophages, another type of immune cell, which then produce their own toxic molecules. These molecules kill more beta cells, which draws in more CD8 T cells, which kill more beta cells. By the time symptoms appear, the majority of beta cells are already gone, and insulin production has dropped to the point where blood sugar can no longer be controlled.
What triggers this immune attack in the first place isn’t fully settled, but it likely requires both genetic susceptibility and an environmental push. Certain gene variants related to immune function raise the risk, though most people who carry them never develop type 1. Viral infections, particularly enteroviruses like Coxsackie B, are the most studied environmental trigger. The link between enteroviruses and type 1 diabetes has been investigated since the 1960s, and evidence continues to build. These viruses may infect or inflame the pancreas directly, or they may set off an immune response that accidentally turns against beta cells. Multiple triggers at different stages of the disease probably need to line up before the clinical onset of type 1 diabetes.
Type 2: Insulin Resistance and Beta Cell Burnout
Type 2 diabetes starts with insulin resistance, a condition where your muscles, liver, and fat tissue stop responding efficiently to insulin. Your pancreas compensates by making more insulin, sometimes for years. Eventually, the beta cells can’t keep up with the demand, insulin production falls, and blood sugar rises.
Excess body fat is the single biggest driver of this process, particularly fat stored around the abdomen and internal organs. Fat tissue isn’t passive storage. When it expands beyond a healthy range, it becomes inflamed and releases inflammatory signals, including TNF-alpha and interleukin-6. These signals activate molecular pathways inside cells that physically interfere with insulin’s ability to do its job. One key pathway blocks an early step in insulin signaling, essentially making your cells deaf to insulin’s message to absorb glucose from the blood.
The major risk factors line up around this mechanism. Being overweight or obese is the most significant. Being physically active fewer than three times a week, having non-alcoholic fatty liver disease, and having a family history of type 2 diabetes all raise your risk as well. Age matters too: the risk climbs after 45, though type 2 is increasingly seen in younger people as obesity rates rise.
The good news is that this process is often reversible in its early stages. About 5-10% of people with prediabetes (blood sugar that’s elevated but not yet in the diabetes range) progress to full type 2 diabetes each year. Over 10 years, about 12.5% of people with prediabetes will develop type 2. But weight loss, regular physical activity, and dietary changes can slow or stop that progression, and in some cases reverse prediabetes entirely.
Gestational Diabetes: Hormones From the Placenta
During pregnancy, the placenta produces a surge of hormones, including estrogen, progesterone, cortisol, placental lactogen, and placental growth hormone. These hormones serve an important purpose: they slightly raise blood sugar to ensure the growing fetus gets enough fuel. But they do this by making the mother’s body more resistant to insulin.
For most women, the pancreas ramps up insulin production to compensate. In gestational diabetes, the pancreas can’t produce enough extra insulin to overcome this resistance, and blood sugar stays elevated. The condition typically develops in the second or third trimester, when placental hormone levels peak, and usually resolves after delivery when those hormones drop. However, having gestational diabetes significantly increases the risk of developing type 2 diabetes later in life.
LADA: A Slow-Burning Autoimmune Form
Latent autoimmune diabetes in adults, or LADA, is sometimes called “type 1.5” because it shares features of both type 1 and type 2. Like type 1, it’s caused by an autoimmune attack on beta cells. Like type 2, it appears in adulthood (typically after age 30) and progresses slowly enough that patients initially respond to oral medications and lifestyle changes.
The key difference is that beta cell destruction in LADA happens gradually rather than rapidly. People with LADA test positive for at least one of the autoimmune antibodies seen in type 1 diabetes, most commonly antibodies against glutamic acid decarboxylase (GAD). Their insulin production declines over months to years rather than weeks, eventually reaching the point where insulin injections become necessary. Because LADA doesn’t look dramatic at first, it’s frequently misdiagnosed as type 2 diabetes. The only reliable way to identify it is through antibody testing.
MODY: Inherited Mutations in Single Genes
Maturity-onset diabetes of the young (MODY) is a group of rare inherited forms of diabetes caused by a mutation in a single gene. At least 14 different genes have been linked to MODY, but two account for the vast majority of cases. One affects an enzyme called glucokinase, which acts as a glucose sensor in beta cells. The other affects a protein called HNF1A that regulates insulin production. Together, mutations in these two genes account for 60-100% of MODY diagnoses.
MODY follows an autosomal dominant inheritance pattern, meaning you only need one copy of the mutated gene from one parent to develop it. This creates a distinctive family tree where diabetes appears in every generation. It typically shows up before age 35, in people who aren’t overweight and don’t have the autoimmune markers of type 1. Many people with MODY are initially misdiagnosed with type 1 or type 2, which matters because the treatment can be very different. Some forms of MODY respond extremely well to a class of oral medications, while others are so mild they require no treatment at all.
Pancreatic Disease and Damage
Any condition that damages the pancreas can destroy enough beta cells to cause diabetes. Chronic pancreatitis is the most common culprit. As the disease progresses, the pancreas becomes increasingly scarred with fibrous tissue, which cuts off blood supply to the insulin-producing islets and leads to progressive beta cell loss. Interestingly, recent research suggests this isn’t always from cells dying outright. In some cases, beta cells undergo a transformation where they change into a different cell type and stop producing insulin.
Cystic fibrosis causes a similar problem. The thick mucus that characterizes the disease damages the exocrine pancreas (the part that produces digestive enzymes), and the resulting inflammation and scarring eventually destroys enough of the surrounding beta cells to reduce insulin production significantly. Acute pancreatitis, particularly severe cases involving tissue death, can also lead to diabetes depending on how much of the pancreas is damaged.
Medications That Raise Blood Sugar
Certain medications can push blood sugar into the diabetic range, and corticosteroids (like prednisone, dexamethasone, and hydrocortisone) are the most common offenders. They cause diabetes through a double hit. First, they directly impair beta cells’ ability to sense glucose and release insulin. Studies show that cortisol can suppress insulin secretion within hours, even before blood sugar changes. Second, they increase insulin resistance in muscles and other tissues by raising fatty acid levels in the blood, which interferes with glucose uptake.
The severity depends on the potency and duration of the steroid. Short courses may cause temporary blood sugar spikes that resolve when the medication stops, while long-term use can produce persistent diabetes. Other medications linked to elevated blood sugar include certain drugs used to treat HIV, some organ transplant rejection medications, and specific psychiatric medications, though corticosteroids remain the most well-documented cause.