What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Endocrine System Disorders: Overview and Mechanisms

Name: Endocrine System Disorders: Overview and Mechanisms
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-09-20

An endocrine system disorder is any disease in which a hormone is produced in the wrong amount, delivered to the wrong target, or met with a target tissue that responds abnormally. Almost every one of these diseases can be traced to a single question: where in the feedback loop did the break occur?

That question matters because it decides what you measure, what you treat, and what you can expect. A dog whose thyroid gland has failed and a dog whose pituitary gland has stopped driving the thyroid gland can look identical in the exam room, yet the first dog needs replacement hormone for life and the second may need imaging of the brain. The same logic separates a cat with diabetes caused by obesity from a cat with diabetes caused by a growth hormone secreting pituitary tumor. Getting the anatomy of the loop right is the difference between treating a number and treating a patient.

The Architecture of a Feedback Loop

Every classic endocrine axis has three tiers. The hypothalamus sits at the top and releases a releasing hormone. The pituitary gland sits in the middle and releases a tropic hormone, meaning a hormone whose job is to stimulate another gland. The peripheral gland sits at the bottom and releases the final effector hormone. The effector hormone then travels back up and shuts off the top two tiers. That return signal is negative feedback.

Take the adrenal axis. The hypothalamus releases corticotropin releasing hormone, or CRH. CRH drives the pituitary to release adrenocorticotropic hormone, or ACTH. ACTH drives the adrenal cortex to release cortisol. Cortisol then suppresses both CRH and ACTH. In dogs, CRH has been measured directly in cerebrospinal fluid, and in healthy dogs cerebrospinal fluid CRH concentrations correlate with plasma ACTH, which confirms that the two tiers are functionally linked [1].

The thyroid axis follows the same shape. The hypothalamus releases thyrotropin releasing hormone, or TRH. TRH drives the pituitary to release thyroid stimulating hormone, or TSH. TSH drives the thyroid gland to make thyroxine (T4) and triiodothyronine (T3). T4 and T3 suppress TRH and TSH.

The pancreatic axis is different in one important way. Insulin is not driven by a tropic hormone from the pituitary. It is driven directly by blood glucose, and by incretin hormones released from the gut after a meal. The best characterized incretins are glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP), both of which amplify glucose-stimulated insulin secretion [2]. This means the pancreas has no pituitary tier, so diabetes is never described as primary or secondary in the pituitary sense. It is described by whether the problem is insulin resistance, beta-cell failure, or both.

Primary, Secondary, and Tertiary Lesions

A primary lesion is in the peripheral gland itself. The gland fails or overproduces on its own, and the pituitary responds appropriately by pushing in the opposite direction. In primary hypothyroidism, the thyroid gland cannot make hormone, so TSH rises as the pituitary tries harder. In a functional adrenocortical tumor, the adrenal gland makes cortisol without waiting for ACTH, so ACTH falls.

A secondary lesion is in the pituitary. The pituitary either fails to send the tropic signal or sends too much of it. In secondary hypothyroidism, the thyroid gland is healthy but idle because TSH is missing. In pituitary-dependent hyperadrenocorticism, a pituitary tumor secretes excess ACTH and the adrenal glands enlarge and overproduce cortisol in response.

A tertiary lesion is in the hypothalamus. This tier is the hardest to prove in practice because hypothalamic releasing hormones are difficult to measure in a living patient. The classic debate is whether Cushing's disease starts in the pituitary or the hypothalamus. In dogs with pituitary-dependent hyperadrenocorticism, cerebrospinal fluid CRH concentrations were significantly lower than in control dogs, and the normal correlation between CRH and ACTH was lost [1]. That finding argues against excessive hypothalamic drive as the primary cause and supports the pituitary adenoma as the driver, with CRH suppression as a consequence of the high cortisol.

The Mechanism-First Table

The table below is the core of this article. Read it left to right and each row tells a complete story: what gland is broken, which hormone is affected, where the feedback loop was cut, what test reveals it, and which species example you will see in practice.

Disorder	Gland	Hormone	Feedback lesion	Hallmark lab test	Veterinary example
Hyperadrenocorticism, pituitary type	Pituitary to adrenal	ACTH, then cortisol	Secondary, excess ACTH	Low-dose dexamethasone suppression test	Canine Cushing's disease
Hyperadrenocorticism, adrenal type	Adrenal cortex	Cortisol	Primary, autonomous secretion	Low-dose dexamethasone suppression test with no suppression	Functional adrenocortical tumor, less common than pituitary type
Hypothyroidism	Thyroid	T4, T3	Primary, gland failure	Total T4 with elevated TSH	Canine hypothyroidism
Diabetes mellitus, type 2 like	Pancreas and peripheral tissues	Insulin	Insulin resistance plus beta-cell decline	Persistent hyperglycemia with elevated fructosamine	Feline diabetes
Pituitary pars intermedia dysfunction	Pituitary intermediate lobe	ACTH and related peptides	Secondary, excess ACTH	Resting ACTH and dexamethasone suppression	Equine PPID
Iatrogenic hyperadrenocorticism	Adrenal cortex, suppressed	Cortisol	Tertiary-like, exogenous steroid	History plus low or suppressed cortisol	Steroid treated dogs
Hypersomatotropism	Pituitary	Growth hormone	Secondary, excess GH	Insulin-like growth factor 1	Acromegalic diabetic cat

Why Cortisol Physiology Differs Between Dogs and Cats

Cushing's syndrome is the clearest example of a species difference in endocrine physiology. Naturally occurring hyperadrenocorticism is extremely common in dogs, with an incidence far greater than in humans, while the incidence in cats is much lower and probably closer to the human rate [3]. Many features of the canine disease resemble human Cushing's, but several changes in dogs are unique and are not observed in other species [3].

The mechanism behind the canine form is usually a pituitary tumor that produces ACTH, which is why the condition is called pituitary-dependent hyperadrenocorticism, or PDH [4]. The excess cortisol then suppresses other pituitary cell types. Dogs with PDH have lower basal growth hormone concentrations and a blunted growth hormone response to stimulation, most likely because glucocorticoids increase somatostatin tone [5]. That suppression reverses after the hyperadrenocorticism is controlled, which was shown when growth hormone responsiveness returned in dogs treated with mitotane or after surgical removal of an adrenal adenoma [6]. Prolactin is also elevated in dogs with PDH, and its response to stimulation is exaggerated [5].

Cats rarely develop spontaneous hyperadrenocorticism, and when they do, the same pituitary tumor mechanism applies [4]. The practical consequence is that a cat with poorly controlled diabetes and a pot belly deserves adrenal testing, because hyperadrenocorticism is one of the underlying conditions that can cause diabetes in cats [7].

Why Thyroid Physiology Differs Between Dogs and Cats

The thyroid axis runs in opposite directions in the two main companion species. Dogs get hypothyroidism. Cats get hyperthyroidism. This is not a coincidence of small numbers. It reflects different disease processes in a gland with the same anatomy.

In canine hypothyroidism, the gland itself fails, so this is a primary lesion. Total T4 is low and TSH is high. The clinical picture is a dog that is lethargic, overweight, and cold intolerant, with skin and coat changes.

In feline hyperthyroidism, a benign thyroid nodule secretes hormone autonomously, so TSH is suppressed by the excess T4. The clinical picture is the opposite: weight loss despite a good appetite, hyperactivity, and often a heart murmur. Cats with endocrine disease in general show measurable differences in routine laboratory panels, and total T4 is one of the parameters used to sort diabetic cats from non-diabetic cats in large laboratory datasets [8].

Species differences also show up in how the thyroid axis interacts with other disease. In the German laboratory study of 129,505 cats, total T4 above the reference interval was one of the parameters compared between poorly controlled and well controlled diabetic cats [8]. The lesson is that thyroid status is part of the workup for almost every endocrine patient, not a separate silo.

Why Insulin Physiology Differs Between Cats and Dogs

Diabetes in cats is a disease of insulin resistance layered on top of beta-cell failure. The majority of diabetic cats have a type 2 like disease that results from a combination of peripheral insulin resistance and a progressive reduction in insulin production [9]. Obesity is the single largest driver. Up to 40 percent of the domestic feline population is overweight or obese, and each excess kilogram of body weight produces roughly a 30 percent decline in insulin sensitivity [10]. An obese, insulin-resistant cat with concurrent beta-cell dysfunction is at risk of progressing to overt diabetes [10].

The genetic contribution is real but modest. A genome-wide association study of 200 diabetic domestic shorthair cats and 399 non-diabetic controls identified four single nucleotide polymorphisms associated with diabetes, and the only gene association previously demonstrated in cats was a polymorphism in the feline MC4R gene [11]. Environmental factors such as obesity and physical inactivity remain the dominant contributors [11].

Dogs are different. Diabetes in dogs is usually insulin dependent from the start, closer to type 1 disease, and the insulin resistance story that dominates feline medicine is less central. This is why the feline literature talks about diabetic remission and the canine literature rarely does. In cats, remission is an attainable goal in the majority of newly diagnosed diabetics, though relapse is common because abnormal glucose homeostasis persists [9][7].

The Glucose Numbers

Blood glucose is reported in two units. In the United States, veterinary laboratories report milligrams per deciliter, written mg/dL. Most of the rest of the world reports millimoles per liter, written mmol/L. To convert mg/dL to mmol/L, divide by 18. To convert mmol/L to mg/dL, multiply by 18. A glucose of 180 mg/dL is 10 mmol/L. A glucose of 360 mg/dL is 20 mmol/L.

Reference intervals vary by laboratory and by analyzer, so the range printed on your patient's report is the range that applies. Do not carry a number from one hospital to another. The one figure that is well anchored in the feline literature is fructosamine. In the German laboratory dataset, diabetes was defined as fructosamine at or above 340 micromol per liter, and poor control was defined as fructosamine above 500 micromol per liter, with well controlled cats falling between 340 and 500 micromol per liter [8]. Fructosamine reflects average glucose over the preceding two to three weeks, which is why it separates a genuinely diabetic cat from a cat that was simply stressed at the clinic.

How These Disorders Are Tested in Practice

Adrenal Testing

The two main tests for canine hyperadrenocorticism are the ACTH stimulation test and the low-dose dexamethasone suppression test. A large study of 1,267 dogs compared the individual components of both tests. The difference between post-ACTH cortisol and baseline cortisol had the strongest diagnostic performance for predicting hyperadrenocorticism, with an area under the receiver operating curve of 0.91, while baseline cortisol alone performed much worse at 0.76 [12]. That is a useful reminder that the delta matters more than the single number.

The low-dose dexamethasone suppression test works on a different principle. A normal dog suppresses cortisol after dexamethasone because the synthetic steroid shuts down the axis. A dog with a pituitary tumor suppresses partially or not at all, and a dog with an adrenal tumor does not suppress. Some dogs have an atypical form of hyperadrenocorticism in which the classic tests are negative but the clinical signs are present, and in those cases stimulation of intermediate steroid precursors such as progesterone by ACTH can reveal the diagnosis [13].

Diabetes Testing

Diabetes is diagnosed by persistent hyperglycemia plus compatible clinical signs, with fructosamine used to confirm that the hyperglycemia is chronic rather than a stress artifact. The 2026 AAHA Diabetes Management Guidelines for Cats cover how to differentiate diabetes from transient or mild hyperglycemia, and they also address how to identify cats at risk of developing diabetes [14]. The guidelines retain clinically relevant material from the 2018 edition and add new findings on monitoring and diet [14].

Pituitary Testing in Horses

Equine pituitary pars intermedia dysfunction, or PPID, is a secondary lesion. The intermediate lobe of the pituitary overproduces ACTH and related peptides, which drives the adrenal cortex. The result is a horse with a long, curly coat that fails to shed, muscle wasting along the topline, and laminitis. Resting ACTH and a dexamethasone suppression test are the standard diagnostic tools.

The Mermaid Decision Path

The flowchart below shows the order in which a clinician reasons through a suspected endocrine disorder, starting from the clinical sign and ending at the feedback lesion.

flowchart TD
    A[Clinical sign] --> B[Measure effector hormone]
    B --> C{Effector high or low}
    C -->|High| D[Measure tropic hormone]
    C -->|Low| E[Measure tropic hormone]
    D --> F{Tropic high or low}
    E --> G{Tropic high or low}
    F -->|High| H[Secondary excess]
    F -->|Low| I[Primary excess]
    G -->|High| J[Primary failure]
    G -->|Low| K[Secondary failure]
    H --> L[Image the pituitary]
    I --> M[Image the gland]
    J --> N[Replace hormone]
    K --> O[Image the pituitary]

Comparative and Clinical Relevance

The comparative angle is where veterinary endocrinology earns its keep. Dogs are the best natural model for human Cushing's disease because the disease is common in dogs and rare in people, and the pituitary tumor biology is similar [4][3]. Cats are a natural model for human type 2 diabetes because the same triad of obesity, insulin resistance, and beta-cell decline drives the disease in both species [10][9][11]. Even the cardiac consequences line up. Cats with newly diagnosed diabetes show decreased diastolic function without systolic dysfunction compared with healthy controls, and in insulin-dependent cats the dysfunction progresses over six months rather than resolving [15]. That mirrors what is seen in human diabetics and laboratory animals [15].

The incretin system is the newest comparative frontier. GLP-1 receptor agonists reduce glycemic variability in diabetic cats, and the insulinotropic effect seen in healthy cats matches what is known from other species [2]. These drugs are widely used in human type 2 diabetes but are not yet approved for feline diabetes [2].

Clinical Relevance, Limitations and Common Mistakes

Three pitfalls account for a large share of endocrine misdiagnosis in small animal practice.

The first is stress hyperglycemia in cats. A frightened cat can mount a blood glucose high enough to look diabetic on a single measurement. Fructosamine solves this because it integrates glucose over weeks rather than seconds. The German dataset used a fructosamine cutoff of 340 micromol per liter to classify cats as diabetic, and that threshold exists precisely because single glucose values are unreliable in this species [8].

The second is iatrogenic Cushing's from steroid administration. A dog on chronic glucocorticoids will have all the clinical signs of hyperadrenocorticism and will suppress on adrenal testing because the axis is already shut down by the drug. Excluding iatrogenic hyperadrenocorticism is a required step before diagnosing the spontaneous disease, and it was explicitly part of the diagnostic workup in a reported atypical case [13]. The history matters more than the test.

The third is insulin resistance confounders. A cat with diabetes that will not respond to insulin may have an underlying condition rather than a dosing problem. Hypersomatotropism, hyperadrenocorticism, and diabetogenic drug administration are all recognized causes of diabetes in cats [7]. Obesity itself is a confounder, since each excess kilogram reduces insulin sensitivity by roughly 30 percent [10]. Before escalating insulin, look for the second disease.

A fourth mistake is reading a single hormone value in isolation. The ACTH stimulation test study showed that the change from baseline carries far more diagnostic weight than the baseline itself [12]. The same principle applies to thyroid testing, where a low T4 in a sick dog may reflect non-thyroidal illness rather than true hypothyroidism.

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Review

Every classic endocrine axis has three tiers: hypothalamus, pituitary, and peripheral gland, with negative feedback from the effector hormone.
Primary means the peripheral gland is broken. Secondary means the pituitary is broken. Tertiary means the hypothalamus is broken.
Cushing's in dogs is usually secondary (pituitary tumor driving the adrenals), which is why it is called pituitary-dependent hyperadrenocorticism [4].
Feline diabetes is type 2 like, driven by obesity and insulin resistance plus beta-cell decline, and remission is possible [10][9].
Fructosamine separates true diabetes from stress hyperglycemia in cats, with 340 micromol per liter as the diagnostic threshold in one large dataset [8].
Iatrogenic Cushing's from steroids must be excluded before diagnosing spontaneous disease [13].
Glucose converts between units by a factor of 18: mg/dL divided by 18 equals mmol/L.

Frequently Asked Questions

What is the difference between primary and secondary endocrine disease?

Primary disease starts in the peripheral gland, and secondary disease starts in the pituitary. In primary hypothyroidism the thyroid cannot make hormone, so TSH rises. In secondary hypothyroidism the thyroid is healthy but idle because the pituitary is not sending TSH.

Why is Cushing's disease so much more common in dogs than in cats?

Naturally occurring hyperadrenocorticism is extremely common in dogs, with an incidence far greater than in humans, while the feline incidence is much lower and probably similar to the human rate [3]. The mechanism in dogs is usually a pituitary tumor producing ACTH [4].

Can a cat have high blood sugar without being diabetic?

Yes. Stress hyperglycemia is common in cats at the clinic. Fructosamine testing distinguishes chronic hyperglycemia from a transient stress spike, and one large laboratory dataset used a threshold of 340 micromol per liter to classify cats as diabetic [8].

What causes diabetes in cats?

Most diabetic cats have a type 2 like disease caused by peripheral insulin resistance combined with progressive loss of insulin production [9]. Obesity is the dominant risk factor, and each excess kilogram reduces insulin sensitivity by about 30 percent [10].

What is iatrogenic Cushing's?

Iatrogenic Cushing's is hyperadrenocorticism caused by giving glucocorticoid medication rather than by a tumor. The clinical signs mimic the spontaneous disease, and excluding this cause is a required step in the diagnostic workup [13].

How do I convert glucose between mg/dL and mmol/L?

Divide mg/dL by 18 to get mmol/L, or multiply mmol/L by 18 to get mg/dL. A glucose of 180 mg/dL equals 10 mmol/L. Always use the reference interval printed on your own laboratory report.