Feline Hypoadrenocorticism (Addison's Disease)

Feline hypoadrenocorticism (Addison's disease) is a rare endocrine disorder caused by deficient production of adrenocortical hormones — primarily glucocorticoids (cortisol) and/or mineralocorticoids (aldosterone) — from the adrenal cortex [3]. It has been recognized in cats since its first description in 1983, yet remains exceptionally uncommon, with only approximately 40 cases documented over the following 40 years [1]. More recently, a multicenter retrospective study identified 41 cases, suggesting that heightened clinical awareness may be improving recognition rates [2]. Because clinical signs are nonspecific, episodic, and often temporarily ameliorated by empirical corticosteroid therapy, diagnosis is frequently delayed or missed [1].

Primary Hypoadrenocorticism The most common form in cats is primary hypoadrenocorticism (Addison's disease), in which destruction of the adrenal cortex leads to deficient production of both mineralocorticoids (aldosterone) and glucocorticoids (cortisol) [3]. The most frequently identified cause is immune-mediated adrenocortical destruction, analogous to the autoimmune adrenalitis seen in dogs and humans [7]. However, other causes documented in cats include:

• ·Acute adrenal necrosis — a dramatic and life-threatening cause, as documented in a case involving bilateral adrenomegaly confirmed by ultrasound and cytology [5]
• ·Infiltrative or neoplastic disease — bilateral adrenal destruction by neoplasia or lymphoma
• ·Granulomatous disease — infectious agents such as Histoplasma spp. or other fungi can infiltrate and destroy adrenal tissue [3]
• ·Iatrogenic causes — following bilateral adrenalectomy performed for treatment of hyperadrenocorticism (e.g., adrenocortical tumors or bilateral adrenalectomy for pituitary-dependent disease) [3]

Pathophysiological Consequences Aldosterone deficiency results in failure of sodium retention and potassium excretion in the renal collecting tubules, leading to hyponatremia and hyperkalemia. The resultant decrease in plasma osmolality reduces circulating blood volume (hypovolemia), causing prerenal azotemia, hypotension, and potentially cardiovascular collapse (Addisonian crisis) [3][7]. Hyperkalemia has direct cardiotoxic effects, causing bradycardia and potentially life-threatening arrhythmias [7]. Glucocorticoid deficiency compounds the hypotension and contributes to gastrointestinal signs, weakness, hypoglycemia, and impaired stress responses [3].

Atypical Hypoadrenocorticism A subset of cats present with glucocorticoid deficiency alone, without electrolyte abnormalities, representing "atypical" hypoadrenocorticism. In the multicenter study, 5 of 41 cats (approximately 12%) had normal serum sodium and potassium concentrations at diagnosis [2]. These cats may be especially difficult to diagnose due to the absence of the classic electrolyte abnormalities.

Breed Predisposition A notable breed predisposition has been identified in British Shorthair cats; in one case series of 11 cats, 6 were British Shorthairs, suggesting a possible genetic susceptibility in this breed [6].

Secondary Hypoadrenocorticism Secondary hypoadrenocorticism results from deficient pituitary ACTH secretion, leading to adrenocortical atrophy and glucocorticoid deficiency; mineralocorticoid function is largely preserved in this form, so electrolyte disturbances are typically absent. This form may arise from pituitary lesions or, most commonly in clinical practice, from abrupt withdrawal of exogenous corticosteroid therapy [3].

Diagnosis requires a high index of clinical suspicion given the rarity and the nonspecific, fluctuating nature of signs [1][3].

Signalment and History Cats of any age may be affected, though most reports describe young to middle-aged cats (range 2–10 years; median ~6.5 years in one series) [6][7]. A history of waxing and waning illness, previous temporary response to empirical corticosteroid or supportive therapy, and the presence of lethargy, anorexia, and weight loss should raise suspicion [1][4].

Serum Electrolytes

• ·Hyponatremia and hyperkalemia are the classic biochemical hallmarks [7]
• ·A sodium-to-potassium (Na:K) ratio < 27 (some references use <24–25) is a key screening indicator; a ratio of 26 was identified in one acutely ill cat [5]
• ·Approximately 88% of cats in one large cohort had abnormal Na and/or K concentrations [2]
• ·However, ~12% of cats may have normal electrolytes ("atypical" Addison's disease), emphasizing the importance of not excluding the diagnosis based on normal electrolytes alone [2]

Serum Biochemistry Panel

• ·Azotemia (elevated BUN and creatinine): Prerenal azotemia is common, resulting from hypovolemia and reduced glomerular filtration rate [7]
• ·Hypoglycemia: Can occur due to glucocorticoid deficiency impairing gluconeogenesis [3]
• ·Hypercalcemia: A recognized association; documented in one acute case alongside azotemia and an abnormal Na:K ratio [5]
• ·Elevated liver enzymes (ALT): May be mildly elevated, reflecting hepatic hypoperfusion [3]
• ·Hypoalbuminemia: May be present in chronic cases with poor nutritional status [3]
• ·Elevated total bilirubin (TBIL): Can occur in severely compromised patients

Hematology

• ·Absence of a stress leukogram — in a genuinely sick cat, the lack of expected neutrophilia, lymphopenia, and eosinopenia (i.e., lack of cortisol effect) is a subtle but important clue [3]
• ·Mild non-regenerative anemia (low HCT): Possible due to chronic illness and reduced erythropoiesis [7]
• ·Lymphocytosis and/or eosinophilia — due to cortisol deficiency removing the normal suppressive effect on these cell lines [3]

ACTH Stimulation Test

• ·The definitive diagnostic test for hypoadrenocorticism [3][7]
• ·Baseline cortisol is measured, synthetic ACTH (cosyntropin) is administered (doses vary; typically 125 µg IV or IM), and post-stimulation cortisol is measured at 30–60 minutes
• ·Diagnosis is confirmed when both baseline and post-ACTH cortisol concentrations are subnormal (typically < 55 nmol/L [< 2 µg/dL] post-stimulation, though feline-specific reference ranges should be used) [6][7]
• ·It is important to note that empirical prednisolone administration prior to testing can interfere with cortisol immunoassay results and should be considered when interpreting results [1]

Adrenal Gland Imaging

• ·Abdominal ultrasonography can demonstrate bilateral adrenal gland enlargement (in acute necrotic or infiltrative disease) or, more commonly, bilateral adrenal gland atrophy (in immune-mediated disease) [5][6]
• ·Adrenal width measurements can be supportive; small adrenal glands are consistent with primary hypoadrenocorticism of immune-mediated origin [6]
• ·In the case of acute adrenal necrosis, moderate to marked bilateral adrenomegaly was documented ultrasonographically, with cytological confirmation [5]

Electrocardiography (ECG)

• ·ECG changes secondary to hyperkalemia may include bradycardia, peaked T waves, prolonged PR interval, wide QRS complexes, or sinoventricular rhythm [3][7]
• ·ECG monitoring is important in acute crisis presentations

Emergency / Acute Addisonian Crisis Management Acute hypoadrenocorticism crisis is a medical emergency requiring immediate stabilization [3][7]:

1. ·Intravenous fluid resuscitation — isotonic saline (0.9% NaCl) is the fluid of choice, simultaneously correcting hypovolemia, diluting hyperkalemia, and replacing sodium deficits. Fluid rates must be individualized; careful monitoring is essential [3]
2. ·Glucocorticoid supplementation — rapid-acting glucocorticoids such as dexamethasone sodium phosphate (0.1–1 mg/kg IV) are preferred acutely, as they do not interfere with cortisol assays if ACTH stimulation testing is still pending [3]. Once diagnosis is confirmed, conversion to prednisolone can follow
3. ·Management of life-threatening hyperkalemia — if severe cardiac arrhythmias are present, additional therapies (e.g., IV calcium gluconate for cardioprotection, glucose ± insulin to shift potassium intracellularly) may be warranted [3]
4. ·Monitoring — continuous ECG, serial electrolytes, blood pressure, and urine output during initial stabilization

Long-Term Mineralocorticoid Replacement

• ·Desoxycorticosterone pivalate (DOCP) — the preferred long-term mineralocorticoid replacement in cats. Administered as a subcutaneous or intramuscular depot injection, it is dosed monthly (initially approximately 2.2 mg/kg, with dose adjustments guided by electrolyte monitoring) [6]
• ·A long-term descriptive case series demonstrated that DOCP combined with prednisolone provided effective control in cats with primary hypoadrenocorticism over follow-up periods exceeding 12 months [6]
• ·Fludrocortisone acetate — an oral mineralocorticoid alternative; historically used in cats but requires twice-daily administration, which may be less convenient than DOCP [3]

Long-Term Glucocorticoid Replacement

• ·Prednisolone — the glucocorticoid of choice for long-term replacement in cats, given its superior oral bioavailability compared to prednisone in this species [6]
• ·Typical starting doses are physiological (e.g., 0.5–1 mg/kg/day), often tapered to the lowest effective dose over time [6][3]
• ·Doses may need to be temporarily increased during periods of stress, illness, or anesthesia ("stress dosing") [3]

Monitoring of Long-Term Therapy

• ·Serum sodium and potassium concentrations should be rechecked regularly (e.g., 2–4 weeks after each DOCP dose adjustment, then every 3–6 months once stable) [6]
• ·Body weight, clinical condition, and signs of over- or under-replacement should be monitored at each visit [6]

Secondary Hypoadrenocorticism

• ·Treatment is primarily with glucocorticoid supplementation alone, as mineralocorticoid function is generally preserved [3]
• ·Gradual dose tapering, rather than abrupt cessation, of corticosteroids in cats on long-term steroid therapy is important to prevent iatrogenic hypoadrenocorticism

The prognosis for feline hypoadrenocorticism, once diagnosed and appropriately treated, appears to be favorable, though long-term data specific to cats remain limited given the rarity of the condition [1].

Survival and Outcome Data

• ·In the largest multicenter retrospective study to date (41 cats), outcome data were reported for the cohort, providing the most comprehensive survival information available [2]. The majority of cats that survived the initial crisis and were established on long-term replacement therapy had positive long-term outcomes [2]
• ·In a case series of 11 cats managed with DOCP and prednisolone, all cats that were compliant with long-term treatment maintained good quality of life over follow-up periods exceeding 12 months, supporting that the disease is manageable with appropriate therapy [6]
• ·In the original landmark case series of 10 cats (Peterson et al., 1989), 7 of 10 cats (70%) were discharged and managed successfully long-term; 3 cats died or were euthanized during the acute crisis, reflecting the serious nature of untreated or late-diagnosed Addisonian crisis [7]
• ·Individual case reports corroborate successful long-term management with appropriate mineralocorticoid and glucocorticoid supplementation [4]

Acute Crisis Mortality

• ·Mortality is most significant during the acute Addisonian crisis, when cardiovascular collapse, severe electrolyte disturbances, and delayed diagnosis contribute to a higher risk of death [7]
• ·Prompt recognition and aggressive supportive care substantially improve survival in the acute phase [3]

Prognostic Considerations

• ·Delay in diagnosis — due to the waxing and waning nature of signs and response to empirical corticosteroids — is a major factor adversely affecting outcomes [1]
• ·Atypical cases (without electrolyte abnormalities) may be especially challenging to diagnose and may have delayed treatment initiation [2]
• ·Underlying cause (e.g., neoplastic infiltration vs. immune-mediated) may influence long-term prognosis independently of hypoadrenocorticism management [3]
• ·The British Shorthair breed predisposition warrants particular clinical vigilance in this population [6]

There are no known preventive measures for naturally occurring primary feline hypoadrenocorticism, as the etiology is most commonly immune-mediated adrenocortical destruction, and the precise immunological triggers remain incompletely understood in cats [1][3].

General Recommendations:

• ·No vaccine exists for this condition, and there is no husbandry or dietary intervention proven to reduce risk [3]
• ·Genetic / breeding considerations — given the apparent breed predisposition in British Shorthair cats, awareness within breeding programs and earlier screening of clinically suspicious individuals of this breed may be warranted, though formal genetic testing is not currently available [6]
• ·Iatrogenic prevention — the most actionable preventive measure is the avoidance of abrupt cessation of long-term exogenous corticosteroid therapy in cats, which can precipitate secondary (iatrogenic) hypoadrenocorticism. Gradual tapering protocols should always be employed [3]
• ·Infectious disease control — in geographic regions where fungal diseases (e.g., histoplasmosis) are endemic, minimizing environmental exposure may reduce one contributory cause of adrenal destruction, though this is not specific to hypoadrenocorticism prevention [3]
• ·Early recognition — while not strictly preventive, clinician and owner education regarding the waxing and waning signs of Addison's disease in cats, and awareness that temporary improvement with empirical corticosteroids does not exclude the diagnosis, may reduce diagnostic delay and prevent fatal Addisonian crises [1]