Interpreting Endocrine Assay Results in Small Animals

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Key Takeaways

• Assay validation is paramount: Utilize only assays validated for the specific target species (canine, feline) to ensure clinically meaningful results; cross-species extrapolation of reference intervals is invalid and can lead to misdiagnosis.
• Dynamic testing is often essential: Basal hormone concentrations are insufficient for definitive diagnosis in many endocrine conditions; provocative or suppression tests (e.g., ACTH stimulation, low-dose dexamethasone suppression) are required to assess endocrine axis function.
• Physiological and pharmacological influences are critical considerations: Stress, concurrent illness (e.g., non-thyroidal illness), and drug administration (e.g., alpha-2 agonists, glucocorticoids) can significantly alter hormone concentrations, necessitating careful review of patient history and clinical context.
• Sample handling and laboratory variability impact interpretation: Hemolysis, lipemia, delayed centrifugation, and improper storage degrade hormone stability, particularly for labile analytes like ACTH; interassay coefficients of variation must be considered for serial monitoring.
• Species-specific physiology dictates interpretation: Significant differences exist in hormone structure, secretory patterns, and receptor physiology between species (e.g., cortisol dynamics in dogs vs. cats, feline thyroxine binding proteins), precluding direct comparison of reference intervals or interpretive thresholds.

Endocrine testing in dogs and cats generates results that are only as useful as the interpretive framework applied to them. Assay methodology, sample handling, biologic variation, and species-specific physiology each influence whether a measured hormone concentration reflects true endocrine status or an artifact of the testing process. This article provides a structured approach to interpreting endocrine assay results in small animal practice, with emphasis on assay validation, dynamic test interpretation, and common diagnostic pitfalls. It is written for practicing veterinarians who order endocrine tests routinely and need to distinguish meaningful results from misleading ones.

The clinical questions addressed here are practical ones. When does a single hormone measurement suffice, and when is a dynamic test required? Why do results from one laboratory not match those from another? How do concurrent illness, drug administration, or sample mishandling alter measured concentrations? The answers require familiarity with both the biology of the endocrine systems being tested and the analytical characteriztics of the assays used to measure them.

At a Glance

Assay Validation and Methodological Principles

Endocrine assays measure hormone concentrations through immunologic or chemical methods, each with distinct performance characteriztics. Radioimmunoassay, immunoradiometric assay, chemiluminescent immunoassay, and enzyme-linked immunosorbent assay are the principal formats used in veterinary diagnostics. These methods differ in sensitivity, specificity, dynamic range, and susceptibility to interference, and results obtained by different methods are not interchangeable.

Validation of an assay for a given species is the foundational requirement for meaningful interpretation. Assay kits developed for human samples may cross-react unpredictably with canine or feline hormones, bind matrix proteins differently, or fail to recognize species-specific hormone variants. Evaluation of endocrine function in equine patients emphasizes that assay kits not validated for the target species may yield results with no clinical meaning, and that reference ranges from other species cannot be used to interpret results. The same principle applies directly to canine and feline endocrinology. Veterinary endocrinology laboratories that validate assays for each species are the preferred submission route.

Validation studies assess several performance characteriztics. Biological specificity is evaluated by stimulating or suppressing the hormone axis and confirming expected changes in measured concentrations. Precision is quantified as intra-assay and interassay coefficients of variation. Recovery after dilution confirms that the assay measures the analyte accurately across a range of concentrations. Validation of a chemiluminescent enzyme immunometric assay for plasma adrenocorticotropic hormone in the dog demonstrates this process, reporting intra-assay coefficients of variation from 4.1 to 8.2 percent and interassay coefficients from 4.6 to 14.8 percent for canine ACTH. The same study found that collection with aprotinin, a protease inhibitor, significantly lowered measured ACTH concentrations, illustrating that sample handling can alter results even when the assay itself is sound.

Species Differences in Endocrine Physiology

Endocrine axes share broad structural homology across mammals, but clinically relevant species differences exist in hormone structure, binding proteins, secretory patterns, and receptor physiology. These differences preclude direct extrapolation of reference intervals or interpretive thresholds between species. The caution raised in considerations on adipose tissue physiology in animal models regarding interspecies differences in endocrine function applies equally to diagnostic testing. What is normal for one species may be frankly abnormal for another.

Cortisol dynamics illustrate this point. Dogs and cats differ in their adrenal responses to stress, ACTH stimulation, and dexamethasone suppression. Feline cortisol secretion shows greater variability and a more pronounced response to handling stress than canine cortisol. Thyroid testing presents similar challenges. Feline total thyroxine concentrations are influenced by binding protein concentrations that differ from those in dogs, and the interpretation of borderline results requires species-specific algorithms.

Physiologic and Pharmacologic Influences on Hormone Measurement

Endogenous hormone concentrations fluctuate with circadian rhythms, stress, feeding status, and reproductive stage. These fluctuations can obscure the distinction between health and disease when a single basal measurement is used. The hypothalamic-pituitary-adrenal axis is particularly labile. Stress from hospitalization, venipuncture, or concurrent illness can elevate cortisol and confound the diagnosis of hyperadrenocorticism or hypoadrenocorticism.

Drug administration represents a major source of interpretive error. Alpha-2 agonists, commonly used for sedation and analgesia, alter glucose homeostasis through effects on insulin and glucagon secretion. Effects of dexmedetomidine on glucose homeostasis in healthy cats documented significant changes in glucose, cortisol, insulin, glucagon, and non-esterified fatty acid concentrations after dexmedetomidine administration in healthy cats. These effects are transient but can be misinterpreted as evidence of diabetes mellitus or other endocrine disease if the drug history is not considered. Glucocorticoid administration, whether therapeutic or accidental, suppresses endogenous cortisol production and invalidates adrenal function testing for days to weeks depending on the preparation used.

Reference Intervals and Biologic Variation

Reference intervals are derived from healthy populations of the same species, and ideally the same age, breed, and reproductive status as the patient. They describe the central 95 percent of values in that reference population, meaning that 5 percent of healthy individuals will fall outside the interval by definition. A result slightly outside the reference interval does not establish disease, and a result within it does not exclude disease. The pretest probability of disease, the magnitude of deviation from the reference interval, and the results of confirmatory testing all contribute to the final interpretation.

Serial monitoring introduces additional complexity. Interassay variation means that two measurements of the same sample on different days will differ even if the true hormone concentration is unchanged. When monitoring response to treatment, the change between measurements must exceed the assay's interassay coefficient of variation to be considered clinically significant. Laboratories should provide their assay performance data so that clinicians can apply this criterion.

Limitations of Postmortem Validation

Endocrine diagnostic tests are often validated against postmortem findings, but this gold standard is itself imperfect. Agreement in histologic assessments of the pituitary pars intermedia in aged horses found only fair agreement among pathologists in determining whether histologic findings were consistent with pituitary pars intermedia dysfunction, with a mean kappa value of 0.34. Postmortem assessment agreed with antemortem endocrine testing 79 percent of the time. This study, while performed in horses, illustrates a general principle: the absence of a perfect reference standard complicates the validation of endocrine tests across species. Clinicians should interpret test performance claims with this limitation in mind.

Dynamic Endocrine Testing: Protocols and Interpretation

Dynamic testing is indicated when basal hormone concentrations fall into equivocal ranges or when clinical suspicion is high despite a normal basal result. The choice of stimulation or suppression test depends on the axis being evaluated, the species, and the specific question being asked. Each protocol carries distinct interpretive rules, and those rules do not transfer across assays or laboratories.

Adrenocorticotropic Hormone Stimulation Testing

The ACTH stimulation test remains the standard for diagnosing hypoadrenocorticism in dogs and for monitoring chronic glucocorticoid therapy. Synthetic ACTH is administered intravenously or intramuscularly, with cortisol measured before and after stimulation. The post-stimulation cortisol concentration is the interpretative endpoint, the baseline value adds little diagnostic information in most cases.

For hypoadrenocorticism, a post-ACTH cortisol below the laboratory's reference threshold supports the diagnosis when paired with compatible clinical signs and electrolyte abnormalities. A normal response excludes the disease with high confidence. In cats, the same test is used but reference thresholds differ, and the response to ACTH is generally more variable. Assay-specific cutoffs must be obtained from the performing laboratory, as validation of the specific assay platform determines the reliability of the reported values.

The test does not reliably distinguish pituitary-dependent hyperadrenocorticism from adrenal-dependent disease. That distinction requires additional testing, typically endogenous ACTH concentration or low-dose dexamethasone suppression testing. The ACTH stimulation test also performs poorly in dogs with mild or early hyperadrenocorticism, where the cortisol response may fall within the reference interval.

Low-Dose Dexamethasone Suppression Testing

The low-dose dexamethasone suppression test is the preferred screening test for hyperadrenocorticism in dogs. Dexamethasone is administered intravenously, with cortisol measured at baseline, 4 hours, and 8 hours. Suppression of cortisol below the laboratory's threshold at either the 4-hour or 8-hour time point rules out hyperadrenocorticism in most cases. Failure to suppress supports the diagnosis but does not differentiate pituitary from adrenal disease.

The 4-hour sample adds interpretive value. Dogs with pituitary-dependent disease often show partial suppression at 4 hours followed by escape at 8 hours, whereas dogs with adrenal tumors typically show no suppression at either time point. These patterns are not absolute, and some dogs with pituitary disease show no suppression at all. The test requires a full 8 hours of hospitalization, which limits its practicality in some practice settings.

Thyroid-Stimulating Hormone Stimulation Testing

TSH stimulation testing is used to diagnose hypothyroidism in dogs when basal thyroid hormone concentrations are equivocal. Recombinant human TSH is administered intravenously, with total thyroxine (T4) measured before and 4 to 6 hours after administration. A blunted post-stimulation T4 response supports hypothyroidism, while a normal response excludes it.

The test is less commonly performed now that endogenous canine TSH measurement is widely available, but it retains value in specific situations. Dogs with non-thyroidal illness may have low basal T4 with normal or low endogenous TSH, and the stimulation test can help clarify thyroid status in these cases. The test is expensive and the drug supply can be intermittent, which limits routine use.

Oral Glucose Tolerance Testing

The oral glucose tolerance test is used to evaluate glucose handling when fasting hyperglycemia is borderline or when reactive hypoglycemia is suspected. Glucose is measured at baseline and at intervals after oral glucose administration. Interpretation requires species-specific reference curves, and the test is sensitive to stress, recent food intake, and the presence of concurrent illness.

In cats, the test is particularly difficult to interpret because of the pronounced stress hyperglycemia that occurs in this species. The endocrine effects of alpha-2 agonists on glucose homeostasis further complicate interpretation when sedation is used during testing. For most clinical purposes, a single fasting glucose concentration combined with fructosamine measurement provides more reliable information than an oral glucose tolerance test.

Interpretation of Basal Hormone Concentrations

Total Thyroxine and Canine TSH

Basal total T4 concentration is a sensitive screening test for hypothyroidism in dogs but has limited specificity. Low total T4 occurs in euthyroid dogs with non-thyroidal illness, in dogs receiving certain medications, and in sighthounds with physiologically low thyroid hormone concentrations. A normal total T4 concentration effectively excludes hypothyroidism, but a low value requires confirmation.

Endogenous canine TSH measurement improves specificity. A low total T4 with a normal or elevated canine TSH supports hypothyroidism, while a low total T4 with a suppressed TSH suggests non-thyroidal illness or drug effect. The combination of low total T4 and elevated canine TSH has high diagnostic accuracy, but approximately 20 to 30 percent of hypothyroid dogs have normal canine TSH concentrations. A normal canine TSH therefore does not exclude the disease.

Free T4 measured by equilibrium dialysis is less affected by non-thyroidal illness and binding protein abnormalities than total T4. It is the preferred confirmatory test when total T4 and canine TSH results are discordant. The assay is technically demanding and not all laboratories offer it.

Cortisol and Endogenous ACTH

Basal cortisol concentration has limited diagnostic value in either direction. A normal basal cortisol does not exclude hypoadrenocorticism, and an elevated basal cortisol does not confirm hyperadrenocorticism. The endogenous ACTH concentration is the key differentiating test once hyperadrenocorticism is confirmed. A suppressed endogenous ACTH supports adrenal-dependent disease, while a normal or elevated concentration supports pituitary-dependent disease.

Endogenous ACTH is highly labile. Samples must be collected into chilled EDTA tubes, centrifuged promptly, and frozen immediately. Hemolysis and delayed processing cause falsely low results. The chemiluminescent assay platforms validated for canine ACTH have improved the practicality of this measurement, but sample handling remains the limiting factor.

Insulin and Glucose Ratios

Fasting insulin concentration is used to evaluate insulinoma and to assess insulin resistance. The test requires a concurrent fasting glucose measurement, and the samples must be handled carefully to avoid hemolysis. Insulin concentrations are reported in different units depending on the laboratory, and the reference interval must be interpreted in the context of the specific assay.

The amended insulin-to-glucose ratio is used to support a diagnosis of insulinoma in dogs. The ratio is calculated from fasting insulin and glucose concentrations, and values above the laboratory's threshold support the diagnosis. The ratio is not diagnostic on its own, and confirmation requires demonstration of hypoglycemia with concurrent hyperinsulinemia. The ratio loses interpretive value in animals with concurrent illness or in those receiving medications that affect glucose homeostasis.

Decision Table for Common Endocrine Assays

Common Pitfalls in Endocrine Assay Interpretation

Stress and Handling Effects

Stress-induced hormone changes are the most common cause of misleading endocrine results in small animals. Cats are particularly susceptible to stress hyperglycemia, and cortisol concentrations rise in any hospitalized patient. The physical examination, venipuncture, and transport to the laboratory all contribute to the stress response. Samples should be collected as quickly as possible after patient arrival, ideally before other procedures are performed.

Drug Interactions

Numerous medications alter hormone concentrations or interfere with assay performance. Glucocorticoids suppress endogenous cortisol and ACTH, and the suppressive effect can persist for weeks after discontinuation. Phenobarbital and sulfonamides lower total T4 concentrations. Alpha-2 agonists such as dexmedetomidine increase blood glucose concentrations, and the effect on glucose homeostasis should be considered when interpreting glucose results from sedated patients.

Assay Platform Differences

Hormone assays are not interchangeable across platforms. A chemiluminescent assay validated for canine ACTH may produce different results than a radioimmunoassay measuring the same hormone. Reference intervals are assay-specific and laboratory-specific, and results should only be interpreted against the reference interval provided by the performing laboratory. Cross-species extrapolation of reference intervals is not valid, as assay validation in the target species is required for meaningful interpretation.

Sample Handling and Storage

Endogenous ACTH is the most fragile of the commonly measured hormones, but other analytes also degrade with improper handling. Insulin is susceptible to hemolysis, and catecholamines require immediate processing. The laboratory's sample requirements should be confirmed before collection, and the practice should have a written protocol for each endocrine assay it submits.

Recognized Complications and Failure Modes

Endocrine assay interpretation fails in predictable patterns. The most common failure mode is overinterpretation of a single hormone value without considering the assay platform, the patient's physiologic state, or the dynamic range of the test. A cortisol concentration in the high-normal range may represent stress, early disease, or assay drift, and the discriminating step is repeat testing under controlled conditions or progression to a dynamic test.

Sample degradation accounts for a substantial proportion of erroneous results. ACTH is particularly labile, and the chemiluminescent assay validated for canine ACTH demonstrates that samples collected with aprotinin yield significantly lower concentrations than those collected without, indicating that protease inhibition alters measured values in a platform-specific manner Validation of a chemiluminescent enzyme immunometric assay for plasma adrenocorticotropic hormone in the dog. Laboratories that use different collection tubes or transport conditions may report discordant results from the same patient. The corrective action is to confirm the laboratory's specific sample handling requirements before collection and to interpret results only against reference intervals generated on the same platform.

Drug effects represent another recognized failure mode. Alpha-2 agonists such as dexmedetomidine produce transient hyperglycemia in healthy cats, with measurable changes in glucose, insulin, and glucagon concentrations that persist for at least 180 minutes after administration Effects of dexmedetomidine on glucose homeostasis in healthy cats. A glucose concentration obtained during or shortly after sedation may therefore misclassify a normoglycaemic cat as diabetic. The discriminating check is to review the sedation record and, when doubt exists, repeat the measurement after recovery.

Common Errors and Corrective Actions

Less experienced clinicians frequently interpret endocrine results against reference intervals from other species. This practice is invalid. Assay kits that have not been validated for the target species may yield results with no clinical meaning, and normal values from one species cannot be applied to another Evaluation of endocrine function. The corrective action is to use a veterinary endocrinology laboratory with species-specific validation and, when such a laboratory is unavailable, to submit age, breed, and sex-matched control samples alongside the patient sample.

A second common error is treating a single borderline value as diagnostic. Endocrine systems are dynamic, and basal hormone concentrations overlap substantially between healthy and diseased animals. The corrective action is to select the test with the best separation for the question being asked, which often means a dynamic test instead of a basal measurement.

A third error is failure to account for biologic variation. Hormone concentrations fluctuate with circadian rhythm, stress, feeding status, and reproductive stage. A single measurement cannot distinguish biologic variation from early disease. Repeat sampling or provocative testing resolves this ambiguity.

Limitations of Current Evidence

The evidence base for endocrine assay interpretation in small animals has important gaps. Postmortem validation of antemortem tests is complicated by poor agreement among pathologists. In aged horses with mild signs of pituitary pars intermedia dysfunction, histologic assessment of the pars intermedia showed only fair inter-pathologist agreement, with a mean kappa value of 0.34 Agreement in histologic assessments of the pituitary pars intermedia in aged horses. If the reference standard itself is unreliable, then the apparent accuracy of antemortem tests may be overstated or understated depending on which pathologist's interpretation is used.

Expert opinion still differs on several interpretation thresholds. The optimal cut-off for dynamic test responses, the clinical significance of mild hormone elevations, and the role of newer assay platforms remain areas of active debate. The ACVIM consensus statements provide structured guidance where evidence permits, but clinicians should recognize that consensus statements reflect expert agreement instead of definitive proof.

Referral and Escalation Criteria

Referral to a specialist is warranted when the diagnostic question remains unresolved after appropriate testing, when dynamic testing carries risk to the patient, or when the clinical signs and laboratory results are discordant. Specialist consultation is also appropriate when the clinician lacks access to a validated assay platform or when interpretation requires integration of multiple endocrine axes.

Laboratory involvement is indicated when results fall outside the expected biologic range, when assay interference is suspected, or when results are inconsistent with the clinical picture. A reputable laboratory can provide platform-specific reference intervals, repeat analyzes, and clarification of assay limitations.

Regulatory reporting applies when endocrine testing intersects with public health or trade requirements. The WOAH terrestrial animal health standards define surveillance and reporting obligations for certain endocrine-related conditions, particularly in production animals. Practitioners should consult their regional veterinary authority for jurisdiction-specific requirements.

Frequently Asked Questions

How do I interpret endocrine results when my clinic only has access to a human hospital laboratory?

Human laboratory assays are frequently not validated for veterinary species, and results may be clinically meaningless. Assay kits that have not been validated for use in dogs or cats can yield spurious values due to differences in hormone structure, binding proteins, and matrix effects. Submit samples to a veterinary endocrinology laboratory with species-specific validation whenever possible. If that is not feasible, collect age, breed, and sex matched control samples from healthy animals and submit them alongside the patient sample to establish a contemporaneous reference frame. Reference ranges derived from human populations cannot be applied to canine or feline samples, as evaluation of endocrine function in equine patients has similarly demonstrated for horses.

What should I do when a dynamic test result conflicts with the clinical picture?

Reconcile the discrepancy before committing to a diagnosis. First, verify sample handling and timing, since delayed centrifugation, hemolysis, or prolonged storage degrades ACTH and other labile analytes. Second, review concurrent medications, including glucocorticoids, phenobarbital, and alpha-2 agonists, all of which alter measured hormone concentrations. Third, consider repeating the test after a washout period. When clinical signs are strong but the assay is equivocal, a second dynamic test targeting the same axis may clarify the diagnosis. Remember that postmortem histopathology is an imperfect gold standard, agreement among pathologists in assessing pituitary pars intermedia lesions was only fair, so antemortem endocrine testing may outperform necropsy in mild disease.

How do I explain endocrine assay limitations to a client who expects a definitive answer?

Frame the assay as one component of a diagnostic picture, not an oracle. Explain that hormone concentrations fluctuate with stress, feeding, and time of day, and that reference intervals describe populations instead of individuals. Use a concrete example, such as a single borderline cortisol value that requires an ACTH stimulation test to confirm. Emphasize that the laboratory measures a chemical quantity, while the diagnosis integrates physical examination, history, and imaging. Reassure the client that repeat testing is routine and does not indicate failure. This approach aligns with the ACVIM consensus statement framework, which emphasizes structured diagnostic reasoning over isolated laboratory values.

When is it acceptable to rely on a single basal hormone measurement instead of a dynamic test?

Basal measurements are acceptable when the pretest probability is extreme. A total thyroxine concentration below the detection limit in a dog with classic hypothyroid signs supports the diagnosis without further testing. A normal total thyroxine in the same dog effectively excludes hypothyroidism. Conversely, a low-normal total thyroxine in a sick or geriatric patient is expected and should not prompt thyroid supplementation. For adrenal disease, a single resting cortisol is useful only as a screening test: a value below the reference interval makes hypercortisolism unlikely, while a high value requires confirmation with dynamic testing. Basal ACTH is reliable only when collected and handled correctly, since protease inhibitors can significantly lower measured ACTH concentrations in some assay systems.

How should I adjust my interpretation when testing a species for which the assay was not originally validated?

Treat the result as investigational, not diagnostic. Cross-species assay use carries substantial risk because antibody binding, reference intervals, and physiologic regulation differ across taxa. The MSD Veterinary Manual advises species-specific interpretation for all endocrine testing. When no validated assay exists, establish a local reference interval using at least 20 healthy individuals of the same species, age, and reproductive status. Interpret the patient result relative to that cohort instead of to the assay manufacturer's human or canine ranges. Document the validation limitations in the medical record. Consider whether a functional test, such as a stimulation or suppression protocol, provides more reliable information than a single hormone concentration in an unvalidated system.

What documentation should accompany endocrine assay submissions to minimize interpretation errors?

Provide the laboratory with species, breed, age, sex, reproductive status, current medications, and the specific clinical question. Note the time of sample collection relative to feeding, exercise, and any drug administration. Record the collection tube type, whether anticoagulant or protease inhibitor was used, and the time from collection to centrifugation and freezing. For dynamic tests, document the exact dose of the stimulating or suppressing agent, the route, and the sampling times. This information allows the laboratory to flag results that may be affected by known interferences. The AVMA practice resources recommend standardized submission forms for endocrine testing to reduce clerical errors and improve result comparability across serial samples.

Related Clinical & Scientific Guides

• Feline Hepatic Lipidosis: Nutritional and Medical Management
• Canine Respiratory Infection: Diagnostic Approach and Treatment
• Canine Respiratory Virus: Diagnostic and Management Considerations

References and Further Reading

• Agreement in histologic assessments of the pituitary pars intermedia in aged horses.. 2005.
• Evaluation of endocrine function.. 1995.
• Of mice and men: Considerations on adipose tissue physiology in animal models of obesity and human studies.. 2022.
• Validation of a chemiluminescent enzyme immunometric assay for plasma adrenocorticotropic hormone in the dog.. 2003.
• Effects of dexmedetomidine on glucose homeostasis in healthy cats.. 2020.
• Interpretable machine learning for the identification of estrogen receptor agonists, antagonists, and binders.. 2024.
• ACVIM Consensus Statements. Journal of Veterinary Internal Medicine.
• MSD Veterinary Manual, Professional Edition. MSD Veterinary Manual.
• American Veterinary Medical Association Practice Resources. American Veterinary Medical Association.

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• Canine Acute Respiratory Distress Syndrome: Diagnostic Criteria and Management

This article is educational professional reference material for veterinary audiences. It is not a substitute for veterinary diagnosis, individual clinical judgment, current product labeling, or applicable regulatory requirements.