Endocrine Testing: Low-Dose Dexamethasone Suppression for Cushing's Disease in Dogs

Introduction

Hyperadrenocorticism (HAC), commonly termed Cushing's syndrome, is one of the most frequently diagnosed endocrine disorders in middle-aged and older dogs [1]. Spontaneous HAC is subclassified into pituitary-dependent hyperadrenocorticism (PDH), caused by a functional corticotroph adenoma of the pars distalis or pars intermedia, and adrenal-dependent hyperadrenocorticism (ADH), resulting from a functional adrenocortical neoplasm [1, 2]. PDH accounts for approximately 80 to 85 percent of spontaneous cases in dogs [1]. The low-dose dexamethasone suppression test (LDDST) is a cornerstone diagnostic procedure designed to discriminate between PDH and ADH while also serving as a confirmatory test for HAC [2, 3]. This article provides a detailed clinical reference on the LDDST, covering its physiologic basis, protocol, interpretive criteria, diagnostic accuracy, and integration with other endocrine testing, including a decision algorithm illustrated with a Mermaid workflow diagram.

Physiologic Basis of Dexamethasone Suppression

The LDDST exploits the negative feedback inhibition of the hypothalamic-pituitary-adrenal (HPA) axis by exogenous glucocorticoids [1]. In a normal dog, administration of a low dose of dexamethasone (0.01 to 0.015 mg/kg intravenously) suppresses pituitary secretion of adrenocorticotropic hormone (ACTH) for 8 to 24 hours, resulting in a measurable decrease in serum cortisol concentration [1, 2]. Cortisol concentrations typically fall below the laboratory reference interval (e.g., less than 1.0 to 1.4 µg/dL) at the 4- or 8-hour sampling point [2, 3].

In dogs with PDH, the neoplastic corticotroph cells retain partial sensitivity to glucocorticoid negative feedback due to expression of glucocorticoid receptors, albeit with a blunted response [1, 2]. As a result, administration of dexamethasone at the same dose typically produces suppression of cortisol at the 4-hour or 8-hour time point, but cortisol concentration may escape from suppression by the 8-hour or 24-hour sample, or suppression may be incomplete [2, 3]. In contrast, adrenal-dependent HAC, arising from a cortisol-secreting adenoma or carcinoma, is autonomous and independent of ACTH regulation [1]. The neoplastic adrenal cells do not express functional glucocorticoid receptors, and exogenous dexamethasone does not affect cortisol secretion [1, 2]. Therefore, cortisol concentrations remain unchanged throughout the test [3].

Test Protocol

The LDDST is performed as a single-day procedure requiring three blood samples and a single intravenous injection of dexamethasone [2, 3]. Standardized protocols are presented in Table 1.

Table 1. Low-Dose Dexamethasone Suppression Test Protocol for Dogs

The injection volume is small (typically 0.1 to 0.3 mL for a 10 to 30 kg dog) and can be administered through an indwelling intravenous catheter or by direct venipuncture [2]. Dexamethasone sodium phosphate is the preferred formulation because it is water-soluble and rapidly absorbed [1]. Serum cortisol is measured by a validated chemiluminescent immunoassay, enzyme-linked immunosorbent assay (ELISA), or radioimmunoassay on an automated analyzer [2, 3]. Samples should be processed within 2 hours or stored at 4 degrees Celsius for analysis within 48 hours [2].

Interpretation of Results

Interpretation of the LDDST relies on absolute cortisol concentrations and suppression patterns. Standard interpretive criteria are summarized in Table 2.

Table 2. LDDST Interpretation Criteria in Dogs

A commonly used threshold for adequate suppression is a serum cortisol concentration of ≤1.4 µg/dL (40 nmol/L) at both the 4-hour and 8-hour time points [2]. Some laboratories use ≤1.0 µg/dL (28 nmol/L) as a stricter criterion [3]. Suppression at 4 hours with escape at 8 hours is highly suggestive of PDH, with a reported positive predictive value of approximately 85 to 90 percent [2, 3]. Complete lack of suppression (all post-dexamethasone cortisol values >1.4 µg/dL) is consistent with ADH but also can occur in a minority of dogs with PDH (approximately 10 to 15 percent) [1, 2]. The LDDST has an overall diagnostic accuracy for discriminating PDH from ADH of 85 to 95 percent in dogs with confirmed HAC [2, 3].

Factors Affecting Test Results

Several factors can influence LDDST results and must be considered in clinical interpretation [1, 2, 3].

• Nonadrenal illness: Concurrent systemic disease (e.g., chronic renal failure, diabetes mellitus, infections) can falsely elevate baseline cortisol and alter suppression dynamics, leading to false-positive or false-negative results [1, 2]. Dogs with significant nonadrenal illness should undergo stabilization before endocrine testing [2].
• Stress: Transportation, hospitalization, and venipuncture stress can stimulate the HPA axis and reduce suppression magnitude [3]. If possible, dogs should be acclimated to the clinic environment and handled gently during blood collection [2].
• Drug interference: Phenobarbital, ketoconazole, trilostane, mitotane, and glucocorticoid-containing medications (including topical and otic preparations) can alter cortisol concentrations or dexamethasone clearance [1, 2]. A washout period of 4 to 6 weeks for mitotane and 1 to 2 weeks for trilostane is recommended before testing [2].
• Physiologic variation: Baseline cortisol concentrations exhibit episodic secretion and can fluctuate; a single elevated baseline value is not diagnostic for HAC [1]. Cortisol also has a mild diurnal variation in dogs, though not as pronounced as in humans [2].

Comparison with Other Diagnostic Tests

The LDDST is often used sequentially with other tests for hyperadrenocorticism.

Table 3. Comparison of Confirmatory Tests for Canine HAC

The LDDST offers the advantage of providing both confirmatory and discriminatory information in a single test, reducing the need for multiple visits [2, 3]. The ACTH stimulation test is the preferred test for monitoring medical therapy (e.g., trilostane) but has lower sensitivity for naturally occurring HAC, especially in dogs with mild disease [1, 2]. The high-dose dexamethasone suppression test (HDDST) uses 0.1 mg/kg dexamethasone and is reserved for cases where LDDST is equivocal or unexpected [2]. Endogenous ACTH measurement is the gold standard for discriminating PDH from ADH but requires specialized handling (plasma frozen immediately, shipping on dry ice) and is more costly [2, 3].

Diagnostic Algorithm

The following Mermaid diagram summarizes a recommended stepwise diagnostic approach integrating LDDST in the workup of suspected canine HAC.

flowchart TD
    A[Clinical suspicion of HAC: PU/PD, pot-bellied, hepatomegaly, alopecia, polyphagia], > B{Perform LDDST}
    B, > C[Interpret cortisol at 0, 4, 8 hours]
    C, > D{Suppression present? <br> (cortisol ≤1.4 µg/dL at 4h or 8h)}
    D, Yes, > E[Consistent with HAC; likely PDH]
    E, > F[Consider endogenous ACTH or HDDST for confirmation of PDH]
    D, No suppression at any time, > G[Cortisol remains >1.4 µg/dL at all points]
    G, > H{ACTH stimulation test or endogenous ACTH}
    H, > I[If ACTH stimulation test abnormal, HAC confirmed; likely ADH]
    H, > J[If endogenous ACTH low: ADH; if intermediate/high: PDH]
    C, > K{Equivocal: e.g., suppression at 4h but escape at 8h?}
    K, Yes, > L[PDH highly likely]
    K, No, but clinical signs strong, > M[Repeat LDDST in 4-6 weeks, or perform alternative test]

Frequently Asked Questions

What is the specific mechanism by which dexamethasone suppresses cortisol in PDH dogs?

The mechanism involves binding of dexamethasone to glucocorticoid receptors on neoplastic corticotroph cells, inhibiting proopiomelanocortin (POMC) transcription and reducing ACTH secretion, which in turn lowers adrenal cortisol production [1, 2].

How often does the LDDST give false-negative results?

False-negative results (normal suppression in a dog that truly has HAC) occur in approximately 5 to 15 percent of cases, often due to a functioning adrenal tumor or a PDH adenoma that has lost glucocorticoid receptor sensitivity [2, 3].

What is the recommended dexamethasone dose for the LDDST?

The standard dose is 0.01 mg/kg dexamethasone sodium phosphate intravenously, although some protocols use 0.015 mg/kg or even 0.015 mg/kg as an alternative [1, 2, 3]. Doses higher than 0.015 mg/kg may suppress PDH and ADH alike, reducing discriminatory power [2].

Should owners withhold food or water before the LDDST?

There is no requirement for fasting; the test is non-oral and food intake does not affect intravenous dexamethasone absorption [2]. However, access to water is typically maintained.

How do concurrent medications affect LDDST results?

Phenobarbital induces hepatic microsomal enzymes that accelerate dexamethasone metabolism, potentially causing insufficient suppression and false suspicion of ADH [1]. Ketoconazole and trilostane lower cortisol production and may produce false suppression [2]. Glucocorticoid-containing drugs (including topical otic preparations) cross-react in cortisol assays and must be discontinued [2].

Can the LDDST be used to monitor therapy for Cushing's disease?

No, the LDDST is not suitable for monitoring treatment with trilostane or mitotane because those drugs act on adrenal steroidogenesis, not on pituitary suppression [1, 2]. The ACTH stimulation test is the standard monitoring tool for medical therapy [2].

References

[1] Feldman EC, Nelson RW. Canine and Feline Endocrinology. 4th ed. St. Louis: Elsevier Saunders; 2015.

[2] Behrend EN, Kooistra HS, Nelson R, Reusch CE, Scott-Moncrieff JC. Diagnosis of spontaneous canine hyperadrenocorticism: 2012 ACVIM consensus statement (small animal). Journal of Veterinary Internal Medicine. 2012;26(3):493-506.

[3] Merck Veterinary Manual. Hyperadrenocorticism (Cushing's Syndrome) in Dogs. Kenilworth: Merck Sharp & Dohme Corp.; 2023. Accessed August 2023. Available at: https://www.merckvetmanual.com/endocrine-system/the-pituitary-gland/hyperadrenocorticism-in-dogs. *** Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.