Diabetes mellitus (DM) is an endocrine disorder characterized by abnormalities in glucose metabolism related to insulin deficiency, reduced insulin action, or a combination of these factors.
Although the name is the same, the underlying mechanism varies significantly from one patient to another, and this also affects the diagnostic process. In dogs, insulin deficiency is the predominant form, with a clinical picture similar to that of human type I diabetes. In cats, insulin resistance associated with progressive beta-cell dysfunction is more common, resembling the profile of type II diabetes. Therefore, before ordering tests, it’s important to understand which of these conditions the patient is most likely to have.
What are the clinical signs of diabetes mellitus?
The most common signs of diabetes mellitus are:
- polydipsia;
- polyuria;
- polyphagia;
- weight loss, even with a normal or increased appetite.
In dogs, cataracts may be associated with the disease; in cats, diabetic neuropathy may occur. None of these signs should be evaluated in isolation: medical history, physical examination, and laboratory tests guide the investigation. Medications, hormonal changes, obesity, pancreatitis, and other concomitant diseases also interfere with insulin action and glucose metabolism.
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Type 1 diabetes: insulin deficiency
In type 1 diabetes, the pancreatic beta cells stop producing enough insulin. It is the most common form in dogs and can also affect cats, especially as beta cell loss progresses over time. Without insulin, glucose cannot enter the cells; it accumulates in the blood and is excreted in the urine. In more severe cases, the body turns to fat as an energy source, and ketones appear.
How is diabetes mellitus diagnosed?
The reasoning here is straightforward and is based on two routine tests: glucose and urinalysis. Persistent hyperglycemia with glycosuria in a patient with compatible clinical signs confirms the diagnosis. In more severe metabolic conditions, the urine may also reveal ketones, which indicates greater urgency.
Scientific evidence
The 2026 AAHA guidelines for dogs emphasize that documenting glycosuria with concomitant hyperglycemia establishes the diagnosis of diabetes mellitus, and that clinical diabetes must be distinguished from transient causes of insulin resistance.
Does hyperglycemia mean diabetes?
Not necessarily. Stress, diet, medications, and other conditions can also raise blood glucose levels, and hyperglycemia alone is not sufficient for a diagnosis. In cats, the stress of handling and sample collection can cause transient elevations, which requires even greater caution. The question, therefore, is not just “Is the blood glucose level high?” but “What does this change mean in the clinical context of this patient?”
Type 2 diabetes: insulin resistance
In type II, insulin is present but does not work as it should. The pancreas tries to compensate, but over time, the beta cells become exhausted and secretion can no longer keep up with demand. This is the most common type in cats, often linked to obesity and inactivity. In dogs, insulin resistance usually develops secondarily, associated with conditions such as diestrus, acromegaly, hypercortisolism, and the use of glucocorticoids.
How is the diagnosis made?
In this context, glucose alone does not tell the whole story. It is common to measure glucose and insulin together: glucose shows the circulating concentration at that moment. Insulin helps assess the hormonal response that regulates that glucose. Together, they allow us to investigate insulin resistance and inadequate insulin secretion, especially when glucose alone does not explain the clinical picture.
Glucose represents blood glucose levels; insulin helps us understand them.
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Interpretation begins before the result is known
Assessing diabetes mellitus requires more than just looking at a number on a lab report. Glucose, urinalysis, insulin, clinical signs, medical history, medications, diet, and comorbidities all contribute to the diagnostic process. When a more in-depth evaluation of hormonal response is needed, the relationship between glucose and insulin provides additional information.
Hormonal diagnosis requires a method, context, and interpretation.
Bibliographic References
- American Animal Hospital Association (AAHA). 2026 AAHA Diabetes Management Guidelines for Dogs. 2026. aaha.org/diabetes-management-dogs
- Niessen SJM, Bjornvad C, Church DB, et al. Agreeing Language in Veterinary Endocrinology (ALIVE): diabetes mellitus, a modified Delphi method-based system for developing consensus disease definitions. Veterinary Journal. 2022;289:105910. doi.org/10.1016/j.tvjl.2022.105910
- Viau P, Martins F, Dos Santos R, Cattaruzzi D. Evaluation of Serum Insulin Measurement in Dogs: A New Perspective. ACVIM Forum Research Abstract Program. J Vet Intern Med. 2025. doi.org/10.1111/jvim.70258
- Pöppl AG. Hormonal Guide. 2nd ed. Hormonal.
FAQ
Which test confirms diabetes mellitus in dogs?
In dogs, the diagnosis is based on persistent hyperglycemia associated with glycosuria and compatible clinical signs, always taking the patient's history into account.
Does isolated hyperglycemia confirm diabetes?
No. It is necessary to consider clinical signs, glycosuria, and possible causes of transient elevations in blood glucose levels, such as stress.
Why measure insulin along with glucose?
Because the two markers provide different information. When evaluated together, they link circulating glucose to the hormonal response and help investigate insulin resistance and inadequate insulin secretion.
What is the difference between type 1 and type 2 diabetes?
In Type I, there is a lack of insulin, and the diagnosis is made through blood glucose and urinalysis. In Type II, insulin does not work as it should, and in this case, it is common to test both blood glucose and insulin levels. Dogs usually have a form similar to Type I, and cats, to Type II.