NAVLE Endocrinology High-Yield Guide: Hormonal Disease Questions Decoded

Endocrinology questions are among the most consistently tested topics on the NAVLE. Cushing's disease, Addison's disease, hypothyroidism, hyperthyroidism, and diabetes mellitus appear on virtually every exam cycle, as do equine PPID and bovine metabolic disease. These cases reward candidates who have mastered the diagnostic algorithm and know the first-line drug cold. This guide organizes the high-yield clinical information you need — species by species, disease by disease — so you can answer NAVLE endocrinology questions with confidence.

For a broader framework of how endocrinology fits into the full exam, see the NAVLE Exam Complete Guide.

1. Canine Hyperadrenocorticism (Cushing's Disease)

Canine hyperadrenocorticism (HAC) is one of the highest-yield endocrine conditions on the NAVLE. Know the breakdown: PDH (pituitary-dependent hyperadrenocorticism) accounts for approximately 85% of cases; adrenal tumor (AT) accounts for the remaining 15%.

Classic Clinical Signs

The hallmark presentation includes polyuria and polydipsia (PU/PD), pot-belly abdomen (muscle wasting + hepatomegaly), bilaterally symmetric alopecia, calcinosis cutis, hyperpigmentation, comedones, and generalized muscle wasting. Key clinicopathologic changes include a stress leukogram (mature neutrophilia, lymphopenia, eosinopenia, monocytosis), markedly elevated ALP (steroid hepatopathy), and dilute urine specific gravity.

Diagnostic Tests

The low-dose dexamethasone suppression (LDDS) test is the preferred screening test. Administer 0.01 mg/kg dexamethasone IV, then collect cortisol at baseline, 4 hours, and 8 hours. In PDH, cortisol typically suppresses at 4 hours or 8 hours; in AT, there is no suppression at either time point. The high-dose dexamethasone suppression (HDDS) test uses 0.1 mg/kg and helps differentiate PDH (suppresses) from AT (does not suppress). The ACTH stimulation test is the gold standard for monitoring treatment with trilostane or mitotane, not for initial diagnosis.

Treatment

Trilostane (Vetoryl) is the first-line medical treatment — it is a reversible inhibitor of adrenal steroidogenesis. Mitotane (o,p'-DDD, Lysodren) causes adrenocorticolysis and is an alternative, particularly for AT management. Monitor with ACTH stimulation test 4 weeks after starting treatment, then every 3 months. Target post-ACTH cortisol of 2-5 mcg/dL with trilostane.

2. Canine Hypoadrenocorticism (Addison's Disease)

Addison's disease is called "the great pretender" because its vague, episodic presentation mimics many other illnesses. Young to middle-aged dogs (female predisposition) — Standard Poodles, Portuguese Water Dogs, Nova Scotia Duck Tolling Retrievers — are classic NAVLE breeds.

Typical vs. Atypical Addison's

Typical Addison's involves loss of both mineralocorticoids (aldosterone) and glucocorticoids (cortisol), resulting in hyponatremia and hyperkalemia. The Na:K ratio falls below 27 (normal 27-40). Atypical Addison's involves glucocorticoid deficiency only — electrolytes are normal, making it harder to recognize.

Diagnosis and Crisis Management

The ACTH stimulation test is diagnostic: both pre- and post-ACTH cortisol are low (less than 2 mcg/dL post-ACTH). During an Addisonian crisis, treat with IV 0.9% NaCl (volume replacement is critical), dextrose if hypoglycemic, and dexamethasone sodium phosphate IV — dexamethasone does not cross-react with cortisol assays, allowing the ACTH stim test to proceed simultaneously.

Long-Term Management

DOCP (desoxycorticosterone pivalate) IM every 25-28 days replaces mineralocorticoid function. Daily oral prednisolone replaces glucocorticoid function. Stress dosing (2-3x the maintenance prednisolone dose) is required during illness or surgery. Fludrocortisone acetate can replace both mineralocorticoid and glucocorticoid in some patients.

3. Canine Hypothyroidism

Hypothyroidism is the most common endocrine disease in dogs. High-risk breeds include Golden Retrievers, Doberman Pinschers, Labrador Retrievers, and Boxers. The classic patient is a middle-aged, neutered dog with progressive weight gain and lethargy — without the PU/PD that points toward Cushing's.

Clinical Signs

Weight gain, exercise intolerance, cold intolerance, bilateral symmetric alopecia (non-pruritic, trunk and tail), myxedema (tragic facies), hyperpigmentation, and bradycardia (important differentiating feature from Cushing's, which causes tachycardia). Neurologic signs (peripheral neuropathy, vestibular disease) can occur in severe cases.

Diagnosis

Total T4 (tT4) is the initial screening test — a normal result essentially rules out hypothyroidism. For confirmation or equivocal cases, use free T4 by equilibrium dialysis (fT4 by ED) plus canine TSH (cTSH). A low fT4 plus elevated cTSH is diagnostic. Note that many concurrent illnesses and drugs (glucocorticoids, sulfonamides, phenobarbital) suppress T4 (euthyroid sick syndrome), so clinical context matters.

Treatment

Levothyroxine (T4, Soloxine) at 0.02 mg/kg PO q12h is the treatment of choice. Recheck tT4 4-6 hours post-pill at the 4-6 week recheck. Target tT4 in the upper half of the reference range at peak (4-6hr post-pill). Most dogs show clinical improvement within 4-8 weeks; full response may take 3-6 months.

4. Feline Hyperthyroidism

Feline hyperthyroidism is the most common endocrine disease in cats — nearly exclusively in cats over 10 years of age. The underlying pathology is a benign functional adenoma or adenomatous hyperplasia of one or both thyroid lobes.

Clinical Signs

Weight loss despite a ravenous (or increased) appetite, palpable thyroid nodule (ventral cervical region), tachycardia, hypertension, vomiting/diarrhea, and unkempt coat. This is essentially the opposite profile from hypothyroidism. Note that some older cats show an atypical "apathetic" form with lethargy and anorexia.

Diagnosis

Total T4 (tT4) is diagnostic — a markedly elevated result in a cat with compatible signs confirms hyperthyroidism. For borderline cases, free T4 by equilibrium dialysis or a technetium-99m pertechnetate scan (identifies ectopic thyroid tissue) is used.

Treatment Options and the Renal Masking Problem

Four options exist: (1) Methimazole (or carbimazole, a prodrug) — blocks thyroid hormone synthesis; (2) Radioactive iodine (I-131) — curative in over 95% of cases; (3) surgical thyroidectomy; (4) Iodine-restricted Y/D diet (Hill's y/d) — dietary management only if strictly fed.

The critical NAVLE concept is masked renal disease. Hyperthyroidism elevates cardiac output and GFR, artificially keeping creatinine in the normal range despite underlying chronic kidney disease. Treat hyperthyroidism gradually with methimazole and recheck renal values 4 weeks after achieving euthyroidism before committing to definitive therapy.

5. Diabetes Mellitus — Canine and Feline

Canine and feline diabetes mellitus are both NAVLE high-yield topics, but the management differs substantially between species.

Canine Diabetes Mellitus

Predisposed breeds include Samoyeds, Miniature Schnauzers, and Miniature/Toy Poodles. Concurrent conditions — pancreatitis, Cushing's disease, and diestrus (progesterone-driven insulin resistance in intact females) — are frequently tested. Treatment: Vetsulin (porcine lente insulin) or NPH, given BID with meals. Monitor long-term control with fructosamine (reflects average glucose over 2-3 weeks). Diabetic ketoacidosis (DKA) requires IV 0.9% NaCl, a CRI of regular (soluble) insulin, and potassium supplementation (add KCl once urine output confirmed).

Feline Diabetes Mellitus

Burmese cats are overrepresented. High-carbohydrate dry food promotes insulin resistance. Treatment: glargine (Lantus) or PZI (protamine zinc insulin) — both are long-acting and appropriate for cats. Avoid NPH in cats (duration too short). Feed a low-carbohydrate canned diet. Diabetic remission occurs in approximately 50% of cats within 6 months with optimal management (tight glycemic control + dietary change). Monitor with fructosamine or continuous glucose monitoring.

6. Equine Endocrine Disease: PPID and EMS

Equine endocrinology is consistently tested on the NAVLE. The two major conditions — PPID (pituitary pars intermedia dysfunction) and EMS (equine metabolic syndrome) — are distinct diseases that both cause laminitis and are frequently confused.

PPID (Equine Cushing's Disease)

PPID results from a dysfunction/adenoma of the pars intermedia of the pituitary gland. It is most common in horses over 15 years old. The pathognomonic sign is hypertrichosis — an abnormally long, curly coat that fails to shed. Other signs: weight loss, muscle wasting (topline), laminitis, PU/PD, hyperhidrosis, and immunosuppression (recurrent infections, parasite burden).

Diagnosis: elevated resting plasma ACTH, interpreted against seasonal reference ranges (ACTH normally rises in late summer/fall — use the appropriate seasonal threshold). The TRH stimulation test increases diagnostic sensitivity in early PPID.

Treatment: pergolide mesylate (a dopamine agonist) is the only approved treatment; it suppresses the overactive pars intermedia. Lifelong therapy is required. Laminitis management, dental care, and parasite control are important supportive measures.

EMS (Equine Metabolic Syndrome)

EMS is a disorder of insulin dysregulation, not a pituitary tumor. It affects "easy keeper" breeds — ponies, Morgans, Paso Finos, Andalusians, and some Warmbloods. Regional adiposity (cresty neck, supraorbital fat, tailhead fat) and recurrent pasture-associated laminitis are the hallmarks. Hyperinsulinemia is the key diagnostic finding (resting or dynamic insulin response testing).

Management centers on dietary restriction: limit non-structural carbohydrates by soaking hay, eliminating grain and pasture access, and increasing exercise. Metformin has limited evidence in horses but is sometimes used. Levothyroxine can be used short-term to promote weight loss and improve insulin sensitivity.

7. Bovine Endocrine and Metabolic Disease

Bovine endocrine questions on the NAVLE focus on periparturient metabolic diseases. The three highest-yield conditions are milk fever, ketosis, and fat cow syndrome.

Milk Fever (Periparturient Hypocalcemia)

Milk fever occurs at or shortly after calving (within 72 hours), typically in high-producing dairy cows in their third or later lactation. The sudden demand for calcium in colostrum and milk overwhelms homeostatic mechanisms. Classic presentation: "downer cow" — the animal progresses through three stages: excitement/tetany (Stage I) ? lateral recumbency, muscle weakness, cold extremities (Stage II) ? coma (Stage III).

Treatment: IV calcium gluconate (slow IV infusion, monitoring heart rate and rhythm for bradycardia/arrhythmia). Oral calcium boluses are used for prevention and mild cases. Prevention: anionic salt (DCAD) diets prepartum shift the cow to mild metabolic acidosis, upregulating calcium mobilization pathways before calving.

Ketosis (Type I and II)

Ketosis results from a negative energy balance in early lactation. Clinical signs include decreased milk production, anorexia (often selective — eats hay but not grain), sweet/fruity odor to breath and milk, and neurologic signs in nervous ketosis. Diagnosis by ketone testing (beta-hydroxybutyrate in blood, urine, or milk). Treatment: IV 50% dextrose, propylene glycol (oral glucose precursor), and glucocorticoids (dexamethasone or flumethasone) to stimulate gluconeogenesis.

Fat Cow Syndrome

Fat cow syndrome (hepatic lipidosis) results from excessive body condition score at calving (BCS >3.5 in a 5-point scale). Mobilized NEFA overwhelms hepatic oxidative capacity, leading to fatty infiltration of the liver. It predisposes to ketosis, metritis, displaced abomasum, and immune suppression. Prevention by maintaining appropriate BCS at dry-off is the cornerstone.

8. Growth Hormone, IGF-1 Axis, and Dwarfism Questions

While less frequently tested than the diseases above, growth hormone disorders appear in NAVLE question banks. German Shepherd pituitary dwarfism results from a cystic Rathke's pouch (craniopharyngioma), causing deficiency of GH and secondary deficiency of TSH and ACTH. The classic presentation is proportionate dwarfism, retained puppy coat (secondary hairs only), bilateral symmetric alopecia, and hyperpigmentation. IGF-1 levels are low. Treatment with porcine GH or progestins (which stimulate GH release) can temporize signs. Acromegaly in cats (GH excess from a pituitary tumor) causes insulin-resistant diabetes mellitus, prognathia inferior, and organomegaly — recognize it as a cause of refractory feline diabetes.

NAVLE Endocrinology Diagnostic Approach — Step-by-Step

Frequently Tested Endocrine Associations

• Cushing's dog — high ALP (steroid hepatopathy) is almost universal; dilute urine USG; stress leukogram; skin: calcinosis cutis and comedones
• Addison's dog — Na:K <27 (typical); ACTH stim shows cortisol <2 mcg/dL pre and post; bradycardia from hyperkalemia; dexamethasone safe to give before test
• Hypothyroid dog — bradycardia (not tachycardia); myxedema coma possible; diagnose with low fT4 + elevated cTSH
• Hyperthyroid cat — tachycardia; palpable ventral neck nodule; masked CKD; T4 diagnostic; gradual treatment required
• DKA dog/cat — acidosis + ketonemia; regular insulin CRI; 0.9% NaCl; K+ supplementation once urinating
• PPID horse — hypertrichosis pathognomonic; seasonal ACTH rise in fall (adjust cutoffs); pergolide for life
• Milk fever cow — periparturient; downer cow; slow IV calcium gluconate; monitor heart rate

For species-specific canine content including dermatology and neurology, see the NAVLE Canine High-Yield Guide. For feline high-yield topics, see the NAVLE Feline High-Yield Guide. Drug mechanisms and pharmacology appear in the NAVLE Pharmacology High-Yield Guide.

Frequently Asked Questions About NAVLE Endocrinology

What is the best screening test for canine hyperadrenocorticism (Cushing's disease)?

The low-dose dexamethasone suppression (LDDS) test is the preferred screening test for canine HAC. Administer 0.01 mg/kg dexamethasone IV, collect cortisol at 0, 4, and 8 hours. Failure to suppress below 1.4 mcg/dL at 8 hours is diagnostic. The ACTH stimulation test is the gold standard for monitoring trilostane or mitotane treatment, not for initial screening.

How do you distinguish PDH from an adrenal tumor on the LDDS test?

On the LDDS test, PDH typically shows suppression at the 4-hour or 8-hour sample (cortisol falls to less than 50% of baseline or below 1.4 mcg/dL). An adrenal tumor shows no suppression at either time point. The HDDS test (0.1 mg/kg dexamethasone IV) further differentiates: PDH suppresses; AT does not. Endogenous ACTH measurement is also helpful — it is elevated in PDH and low to undetectable in AT.

What electrolyte pattern is classic for typical Addison's disease?

Typical Addison's disease causes loss of both mineralocorticoids and glucocorticoids, producing hyponatremia and hyperkalemia. The Na:K ratio falls below 27 (normal range 27-40). Atypical Addison's involves glucocorticoid deficiency only, so electrolytes remain normal — a critical NAVLE distinction.

Which insulin is preferred for diabetic cats, and why?

Glargine (Lantus) or PZI (ProZinc) are preferred for diabetic cats because they are long-acting with flat profiles, matching the cat's slower insulin clearance. NPH has too short a duration of action in cats. Cats managed with glargine plus a low-carbohydrate canned diet achieve diabetic remission in approximately 50% of cases within 6 months.

What is the pathognomonic sign of PPID (equine Cushing's disease)?

Hypertrichosis — a long, curly, abnormally dense coat that fails to shed normally — is pathognomonic for PPID. It results from loss of dopaminergic inhibition of the pars intermedia. Diagnosis is confirmed by elevated resting plasma ACTH using seasonal reference ranges (ACTH rises naturally in fall, so cutoffs are higher between August and October).

How is feline hyperthyroidism treated, and what renal concern must you monitor?

Feline hyperthyroidism is treated with methimazole, radioactive iodine (I-131), surgical thyroidectomy, or iodine-restricted Y/D diet. The critical NAVLE concept: hyperthyroidism increases GFR and masks underlying chronic kidney disease. After starting treatment, recheck renal values at 4 weeks before committing to definitive therapy (I-131 or surgery), as some cats develop azotemia once hyperthyroidism is controlled.