Cats will generally show a gradual onset of the disease over a few weeks or months, and it may escape notice for even longer.

The first outward symptoms are a sudden weight loss (or occasionally gain), accompanied by excessive drinking and urination; for example, cats can appear to develop an obsession with water and lurk around faucets or water bowls. Appetite is suddenly either ravenous (up to three-times normal) or absent. These symptoms arise from the body being unable to use glucose as an energy source.

A fasting glucose blood test will normally be suggestive of diabetes at this point. The same home blood test monitors used in humans are used on cats, usually by obtaining blood from the ear edges or paw pads. As the disease progresses, ketone bodies will be present in the urine, which can be detected with the same urine strips as in humans.

In the final stages, the cat starts wasting and the body will breaking down its own fat and muscle to survive. Lethargy or limpness, and acetone-smelling breath are acute symptoms of ketoacidosis and/or dehydration and is a medical emergency.

Untreated, diabetes leads to coma and then death.

Too little insulin over time can cause tissue starvation (as glucose can't reach the brain or body). In combination with dehydration, fasting, infection, or other body stresses, this can turn over a few hours into diabetic ketoacidosis, a medical emergency with a high fatality rate, that cannot be treated at home. Many undiagnosed diabetic cats first come to the vet in this state, since they haven't been receiving insulin. Symptoms include lethargy, acetone or fruity smell on breath, shortness of breath, high blood sugar, huge thirst drive. Emergency care includes fluid therapy, insulin, management of presenting symptoms and 24-hour hospitalization.

Low blood sugar is common in persons with type 1 and type 2 DM. Most cases are mild and are not considered medical emergencies. Effects can range from feelings of unease, sweating, trembling, and increased appetite in mild cases to more serious issues such as confusion, changes in behavior such as aggressiveness, seizures, unconsciousness, and (rarely) permanent brain damage or death in severe cases. Moderate hypoglycemia may easily be mistaken for drunkenness; rapid breathing and sweating, cold, pale skin are characteristic of hypoglycemia but not definitive. Mild to moderate cases are self-treated by eating or drinking something high in sugar. Severe cases can lead to unconsciousness and must be treated with intravenous glucose or injections with glucagon.

People (usually with type 1 DM) may also experience episodes of diabetic ketoacidosis, a metabolic disturbance characterized by nausea, vomiting and abdominal pain, the smell of acetone on the breath, deep breathing known as Kussmaul breathing, and in severe cases a decreased level of consciousness.

A rare but equally severe possibility is hyperosmolar hyperglycemic state, which is more common in type 2 DM and is mainly the result of dehydration.

The classic symptoms of untreated diabetes are weight loss, polyuria (increased urination), polydipsia (increased thirst), and polyphagia (increased hunger). Symptoms may develop rapidly (weeks or months) in type 1 DM, while they usually develop much more slowly and may be subtle or absent in type 2 DM.

Several other signs and symptoms can mark the onset of diabetes although they are not specific to the disease. In addition to the known ones above, they include blurry vision, headache, fatigue, slow healing of cuts, and itchy skin. Prolonged high blood glucose can cause glucose absorption in the lens of the eye, which leads to changes in its shape, resulting in vision changes. A number of skin rashes that can occur in diabetes are collectively known as diabetic dermadromes.

Obesity in pets occurs when excessive adipose tissue accumulates in the body, and is generally defined as occurring when an animal's body weight is at least 20% greater than its optimal body weight. Obesity is associated with metabolic and hormonal changes.

Weight gain will occur when an animal is in a positive energy balance, meaning energy provided as calories in the diet exceed calories expended. Evidence suggests that middle-aged cats and dogs, especially those between the ages of 5 and 10, may be at an increased risk of obesity. This is supported by studies showing that as cats age from 2 years to approximately 11.5 years of age their energy requirements decreased. Weight gain will occur if calories from the diet do not decrease with the animal’s energy requirements.

Obesity in pets is usually due to excessive food intake or lack of physical exercise. Owners may view food as a way to reward and treat their pets, which contributes to overfeeding. Pets confined to a house or small yard which are not regularly exercised are more prone to obesity.

The risk of obesity in dogs (but not in cats) is related to whether or not their owners are obese.

In cats, neutering increases the risk of obesity, partly because the alteration in sex hormones after neutering lowers the basal metabolic rate, and partly because neutered cats have a reduced inclination to roam compared to non-neutered cats.

Loss of Pancreatic enzymes leads to maldigestions and malabsorption which may lead to:

- steatorrhea

- weight loss

- fatigue

- flatulence and abdominal distention (bacterial fermentation of unabsorbed food)

- edema (hypoalbuminemia)

- anemia (Vitamin B12, iron, folate deficiency)

- bleeding disorders (Vitamin K malabsorption)

- Metabolic bone disease (Vitamin D deficiency)

- neurologic manifestation

- hypocalcemia

Most affected cats present with muscular weakness and/or ocular signs of hypertension. Signs of muscle weakness can include a plantigrade stance of the hindlimbs, cervical ventroflexion, inability to jump, lateral recumbency, or collapse. Ocular signs of arterial hypertension include mydriasis, hyphema, or blindness due to retinal detachment and/or intraocular hemorrhages. A palpable mass in the cranial abdomen is another potential finding.

Feline hyperaldosteronism is a disease in cats. The symptoms are caused by abnormally high concentrations of the hormone aldosterone, which is secreted by the adrenal gland. The high concentrations of aldosterone may be due directly to a disorder of the adrenal gland (primary hyperadlosteronism), or due to something outside of the adrenal gland causing it to secrete excessive aldosterone (secondary hyperaldosteronism).

One of the reasons a cat may stop eating is separation anxiety and the emotional stress that results. Moving, gaining or losing housemates or pets, going on vacation, or prolonged boarding are all common situations that pet owners report just prior to the onset of the disease, but it may develop without these conditions existing. Obesity is known to increase the risk of hepatic lipidosis; however, there is no known "official" cause of the disease. Severe anorexia usually precedes the onset of the disease. When the cat has no energy from eating, the liver must metabolize fat deposits in the body into usable energy to sustain life. The cat liver, however, is poor at metabolizing fat, causing a buildup of fat in the cells of the liver, leading to fatty liver. At this point the disease can be diagnosed; however, it will often not be diagnosed, and many animals are euthanized due to improper or no diagnosis.

Anorexia always precedes liver disease, with the cat refusing to eat enough food for days, or weeks. This may be amplified by frequent vomiting when the cat does choose to eat. A lack of appetite causes the cat to refuse any food, even after it has purged its system of all stomach contents. Severe weight loss proceeds as the liver keeps the cat alive off body fat, causing a yellowing of the skin (jaundice). When the cat runs out of fat to process, severe muscle wasting (cachexia) takes place as the body converts protein into energy. Eventually the body cannot give the brain enough energy to function properly and the cat dies from malnutrition. In addition, an overworked liver can eventually fail causing total system collapse.

Exocrine pancreatic insufficiency (EPI) is the inability to properly digest food due to a lack of digestive enzymes made by the pancreas. EPI is found in humans afflicted with cystic fibrosis and Shwachman–Diamond syndrome, and is common in dogs. EPI is caused by a progressive loss of the pancreatic cells that make digestive enzymes; loss of digestive enzymes leads to maldigestion and malabsorption of nutrients from normal digestive processes. Chronic pancreatitis is the most common cause of EPI in humans and cats. In dogs, the most common cause is by "pancreatic acinar atrophy", arising as a result of genetic conditions, a blocked pancreatic duct, or prior infection.

The exocrine pancreas is a portion of this organ that contains clusters of ducts (acini) producing bicarbonate anion, a mild alkali, as well as an array of digestive enzymes that together empty by way of the interlobular and main pancreatic ducts into the duodenum (upper small intestine). The hormones cholecystokinin and secretin secreted by the stomach and duodenum in response to distension and the presence of food in turn stimulate the production of digestive enzymes by the exocrine pancreas. The alkalization of the duodenum neutralizes the acidic chyme produced by the stomach that is passing into it; the digestive enzymes serve to catalyze the breakdown of complex foodstuffs into smaller molecules for absorption and integration into metabolic pathways. The enzymes include proteases (trypsinogen and chymotrypsinogen), hydrolytic enzymes that cleave lipids (the lipases phospholipase A2 and lysophospholipase, and cholesterol esterase), and amylase to digest starches. EPI results from progressive failure in the exocrine function of the pancreas to provide its digestive enzymes, often in response to a genetic condition or other disease state, resulting in the inability of the animal involved to properly digest food.

Cat skin disorders are among the most common health problems in cats.

Skin disorders in cats have many causes, and many of the common skin disorders that afflict people have a counterpart in cats. The condition of a cat's skin and coat can also be an important indicator of its general health. Skin disorders of cats vary from acute, self-limiting problems to chronic or long-lasting problems requiring life-time treatment. Cat skin disorders may be grouped into categories according to the causes.

Pork–cat syndrome is an allergy to pork, usually post adolescence, that is caused by exposure to cats. Although first described in 1994, it was first documented in the U.S. by Scott Commins and Thomas Platts-Mills during their research on alpha-gal allergy.

It is called "pork–cat syndrome" because "almost all people with the condition are cat owners, and many have multiple cats. Some develop an allergic response to cat serum albumin (protein made by a cat’s liver) that cross-reacts with albumin in pork when someone consumes it, and can lead to severe or even fatal allergic reactions when pork is consumed."

Skin disease may result from deficiencies in immune system function. In cats, the most common cause of immune deficiency is infection with retroviruses, FIV or FeLV, and cats with these chronic infections are subject to repeated bouts of skin infection and abscesses. This category also includes hypersensitivity disorders and eosinophilic skin diseases such as atopic dermatitis, miliary dermatitis and feline eosinophilic granuloma and skin diseases caused by autoimmunity, such as pemphigus and discoid lupus.

The symptoms can vary but include weight loss, diarrhea, vomiting, and anorexia.

Areas affected are those the cat can access most easily, including the abdomen, legs, flank, and chest.

- Baldness, usually beginning with the abdomen

- Obvious over-grooming (although some cats may only engage in the behavior in the absence of owners)

- Redness, rashes, pus, scabs on the bald area or areas traumatized by over-grooming

- A highly irritable cat may even cut its face with the claw of its hind foot if over-zealously scratching the back of its head.

Psychogenic alopecia, also called "over-grooming" or "psychological baldness," is a compulsive behavior that affects domestic cats. Generally, psychogenic alopecia does not lead to serious health consequences or a decreased lifespan.

Usually symptoms of cerebellar hypoplasia can be seen immediately at birth in cats, but sometimes can take two months or so to become apparent in dogs. Cerebellar hypoplasia causes jerky movements, tremors and generally uncoordinated motion. The animal often falls down and has trouble walking. Tremors increase when the animal is excited and subside when at ease.

The cause of feline hyperesthesia syndrome is unknown. Some experts believe FHS to be a form of epilepsy, while others believe it is a behavioural disorder triggered by trauma. Noting that affected cats tend to be dominating rather than submissive, some research argues that FHS is conflict displacement in which the cat acts out thwarted territorial disputes on its own body.

Although any age, breed, or sex of cat can develop feline hyperesthesia syndrome, those most susceptible include the Siamese, Burmese, and Himalayan breeds.

A squitten is a cat with unusually short forelegs and/or unusually long hind legs that resembles a squirrel. It is an example of a cat body type genetic mutation. The word is a portmanteau of squirrel and kitten.

The term kangaroo cat is also, rarely, used; this derives from a 1953 specimen known as the Stalingrad Kangaroo Cat.

Lymphoma is the most common form of cancer in cats, which is often associated with feline leukemia virus, and accounts for 25 percent of all cases. Feline lymphoma usually strikes the digestive system, causing excessive vomiting and diarrhea. Other common symptoms of lymphoma in cats include swollen lymph nodes, loss of appetite, weight loss, and difficulty breathing.

During an episode cats show a number of typical signs, including skin rolling or twitching, compulsive self-grooming, self-directed pouncing, or aggressive behaviour such as biting or attacking the tail. There may also be pupil dilation, vocalisation and a general increase in activity.

Cerebellar hypoplasia is a disorder found in cats and dogs in which the cerebellum is not completely mature at birth.

Mammary gland tumors are the third common type of cancer in older female cats, with the most common symptom manifesting as a in the breast tissue. The tumor is typically firm and nodular, and adheres to the overlying skin. Nipples may also appear to be red and swollen, oozing yellowish fluid.