Weight loss is a common symptom of hyperthyroidism—or an over-active thyroid, Dr. Cappola says. This means your thyroid—the butterfly-shaped gland in your neck that helps regulate your metabolism and growth—is pumping out too many hormones, resulting in a slew of body changes.

“If I suspected a thyroid issue, I’d probably look for increased hunger or heart palpitations,” she explains. Sleeping problems or feeling hot all the time are also common symptoms of an over-active thyroid, she says.

Dr. Hildreth mentions something experts refer to as the “obesity paradox.” Later in life, weight loss—not weight gain—is associated with a greater risk of death. “As we age, the stomach empties more slowly, which makes you feel fuller longer,” she says. “Also, some of the brain signals that control appetite and fullness become attenuated,” Dr. Hildreth adds. All of this can lead to older adults eating less, losing weight, and failing to get enough nutrients to support their bodies’ needs.

Be sure you’re eating enough protein to help your body carry out important bodily functions, like curbing hunger, stabilizing blood sugar, and building muscle mass—which people lose as they get older. “Many medications can also affect your appetite, so you need to pay attention to how much and how often you’re eating,” Dr. Hildreth adds.

Celiac disease—an autoimmune disorder in which ingesting gluten causes damage to the small intestine—can lead to a drop in weight, and tends to be accompanied by other GI symptoms like bloating and diarrhea, says Jamile Wakim-Fleming, M.D., a gastroenterologist at Cleveland Clinic.

Why? If you have celiac disease and eat gluten, your immune system freaks out a bit. This reaction can mess with the lining of your small intestine, hampering its ability to help you absorb nutrients properly, according to the Mayo Clinic. Inflammatory bowel diseases like Crohn’s disease can also result in unexplained weight loss due to malabsorption as well.

Dr. Ali notes that not feeling well emotionally, stress, or depression can make weight fluctuate to some extent. Loss of appetite is a common side effect of clinical depression and one that can promote unexplained weight loss if you’re unaware that your mood swings are something a bit more serious. “In a lot of cases, the person doesn’t even notice they’re losing weight because they’re mired in the depression,” Dr. Cappola explains. Irritability, heavy drinking, indecision, and problems sleeping are other common symptoms of depression.

Problems with your pancreas, which produces enzymes that aid in digestion, can also lead to unexplained weight loss, Dr. Wakim-Fleming says. People with chronic pancreatitis—a disease in which the pancreas becomes inflamed—tend to lose weight quickly (even if they are eating normally) because the body simply doesn’t produce enough enzymes for proper digestion of food to occur, according to The National Pancreas Foundation. Look for symptoms like stomach pain, discolored (or oily) poop, diarrhea, or nausea after eating fatty foods.

“Especially early on, new-onset diabetes can cause weight loss,” Dr. Cappola says. You may also feel crazy thirsty and notice you’re peeing all the time. “Your body is literally peeing out glucose because you can’t absorb it, and that drives thirst,” she explains. Diabetes also causes your body to suck nourishment from your muscles, which fuels the sudden weight drop.

Dr. Hildreth says inflammatory conditions like rheumatoid arthritis—a condition in which your immune system attacks your healthy tissues by mistake—or some types of infection can knock out a sufferer’s appetite, causing a drop on the scale. These conditions can also cause inflammation in your gut, which could mess with nutrient absorption, leading to unexplained weight loss before diagnosis.

Several types of cancer, as well as a tumor or ulcer in your stomach or intestines, can cause inflammation or malabsorption issues that may lead to a drop in weight, Dr. Wakim-Fleming says. “If someone comes to me with unexplained weight loss, I’ll check their stomach and colon and bowels for tumors or inflammation,” she says. “I’ll als0 look for tumors in the esophagus”—the tube that connects your throat and stomach—“which can make it hard to swallow.” Dr. Ali adds that battling cancer consumes a lot of calories, so the disease itself may be making you unintentionally drop pounds.

Addison’s disease is a condition of the adrenal glands where they don’t produce the right amounts of cortisol and aldosterone. Though not very common, Addison’s disease can cause endocrine-related weight loss that presents itself with thirst, salt cravings, and weakness, says Rekha Kumar, M.D., endocrinologist and associated professor of Clinical Medicine at Weill Cornell. This is because the adrenal glands aren’t functioning properly, which may throw your metabolism off.

Additionally, Addison’s disease impacts the salt and water balance in your body, blood pressure, and your ability to manage stress, says Lydia C. Alexander, M.D., chief medical officer at Enara Health and president-elect of the Obesity Medicine Association. Weight loss can happen due to loss of appetite from nausea or abdominal pain, fatigue, or dehydration, she adds.

As you age, Dr. Kumar says it’s very common to experience muscle loss from lack of physical activity, which can turn into weight loss. This is because muscle is heavier than fat, which can tip the scale lower.

“This is particularly a serious concern in adults over age 65. One of the biggest concerns resulting from unintentional weight loss in this group is Sarcopenia, meaning loss of muscle mass and strength,” warns Dr. Anegawa. “Sarcopenia is associated with worsening functional impairment, disability, falls, and mortality.”

Though weight loss isn’t necessarily a symptom of many autoimmune diseases, if you’re feeling unwell from something like Sjögren's syndrome (a disease that impacts saliva production) or lupus, you may not be eating as you normally would, Dr. Kumar says. Though an autoimmune disease would present itself alongside other symptoms beyond weight loss, she notes.

Those who experience advanced heart failure tend to lose muscle mass, which can lead to weight loss, Dr. Kumar says. This is because the heart is working so hard to function properly, you’re burning more calories than you normally would, she adds.

Dr. Alexander adds, other heart conditions like heart disease, endocarditis, or pulmonary disease can also lead to weight loss. For example, those who have bacterial endocarditis may experience fatigue, fever, and poor appetite, which could cause a loss of weight and muscle mass, she says.

Dr. Kumar says parasites or worms can take up all your calories, so you’ll find that you’re eating as normal but are still dropping pounds unexpectedly.