A horse's ACTH result needs the sampling month, the test protocol, and the laboratory's assay before it can support a PPID decision. For example, a baseline result of 47 pg/mL from a September blood draw falls inside the 30–90 pg/mL interpretive zone in the 2025 Equine Endocrinology Group recommendations. That comparison applies to the Immulite 2000xpi assay and northern-hemisphere calendar. It neither confirms PPID nor proves that the horse is unaffected. The next conversation is about the horse's signs and the interval the laboratory actually applied, rather than an automatic prescription.

The short answer: check three details on the report

Imagine an 18-year-old gelding with delayed shedding whose September result is flagged 'high.' The example is hypothetical, not a patient case or a measured dataset. Before deciding whether the flag reflects disease, ask the clinic to establish these three details:

1. Sampling month and location: use the date the blood was drawn, not the date the report arrived. Northern-hemisphere autumn can change the meaning of a baseline ACTH concentration.

2. Test and sample timing: was this a resting baseline, a sample before TRH, or a sample taken 10 or 30 minutes after TRH? Those results use different decision thresholds.

3. Analyzer, units, and applicable interval: ask which assay measured ACTH and whether the laboratory used a seasonal diagnostic cutoff or a healthy-population reference interval. A range copied from another method may not apply.

Under the EEG baseline table below, 47 pg/mL would be above the PPID-likely threshold from December through June, but inside the interpretive zone in July, August, September, October, and November. This is a comparison with published thresholds, not a diagnosis or a reason to overrule the receiving laboratory. Clinical signs, breed, handling, and the test's purpose still matter.

What ACTH measures in a horse

Pituitary pars intermedia dysfunction (PPID), historically called equine Cushing's disease, is an age-related disorder involving loss of dopamine control over part of the pituitary gland. Affected cells release more ACTH and related peptides. Measuring ACTH in plasma helps veterinarians investigate that process; it does not directly measure every feature of the disease.

PPID becomes more common with age. The EEG estimates a prevalence of about 20% in equids over 15 years and 30% in those over 30. Age raises suspicion, but age alone is not a positive test. The 2022 PPID review by Kirkwood and colleagues describes delayed shedding, a persistent long coat, muscle loss, lethargy, and recurrent infections among the clinical findings. These observations help explain why a veterinarian ordered testing and how much weight to give an indeterminate result.

Equine ACTH interpretation is a different question from canine ACTH stimulation testing, which commonly evaluates cortisol responses. A dog's test timing, cortisol targets, and treatment rules cannot be transferred to this horse's ACTH report.

Month by month: the 2025 baseline ACTH zones

ACTH rises in healthy horses during late summer and autumn, and the rise can be greater in horses with PPID. The EEG October 2025 guidance, Table 3 presents the following baseline decision zones. All pg/mL values below are based on the Immulite 2000xpi analyzer; the months apply to the northern hemisphere. The final column reproduces the rounded SI-unit equivalents from its Table 6.

These are diagnostic decision thresholds, not simply the range containing most healthy horses. The EEG's lower and upper boundaries help balance sensitivity and specificity; results between them require interpretation with the horse's age and clinical findings. A report's generic 'high' flag and the EEG's 'PPID likely' category may therefore answer different questions. Ask the veterinarian which kind of boundary is printed on the report.

The endpoints matter. A baseline value of exactly 40 pg/mL in December–June remains in the published 15–40 interpretive zone; the PPID-likely category is greater than 40. Likewise, 90 pg/mL in September–October remains in the interpretive zone, whereas greater than 90 is in the likely category. Neither category removes the need for clinical judgment.

Keep the unit attached to the number. The table's September–October interpretive zone is 30–90 pg/mL or approximately 6.6–19.8 pmol/L. A result of 47 pg/mL is approximately 10.3 pmol/L; 47 pmol/L is a very different concentration. Unit conversion changes the expression of a value, not the assay or its diagnostic performance.

Geography and age add uncertainty. In the 2025 JAVMA study by Pinn-Woodcock and colleagues, 201 apparently healthy horses were enrolled at nine US locations between June 2019 and May 2020; 154 remained after outlier analysis. ACTH was highest from late August through early October across the studied latitude groups. Horses over 15 had higher values year-round and a larger seasonal increase. The southern group had lower values than the central group. This healthy-horse study supports considering season and age; it does not independently validate every EEG disease cutoff, and effects nearer the equator remain unresolved.

Baseline or stimulated: which result are you reading?

A baseline ACTH test measures the resting concentration. A TRH stimulation test measures ACTH before and after the veterinarian administers thyrotropin-releasing hormone. It can help investigate earlier or subtler PPID when baseline testing is less supportive. Neither test perfectly separates affected and unaffected horses.

The Cornell AHDC protocol specifies a baseline sample and an exactly timed 10-minute post-TRH sample, with a 30-minute sample optional. For an owner reading the report, the useful check is that each sample is identified and interpreted with its matching interval. A time-zero value should not be compared with a post-TRH threshold, and a 30-minute result should not be read against the 10-minute row.

The following post-TRH zones come from the same 2025 EEG tables and retain their Immulite 2000xpi and northern-hemisphere limits. The January–June likely categories support diagnosis in context; they do not establish disease with certainty.

Autumn TRH testing is harder to interpret because healthy horses can have variably exaggerated responses. The EEG does not describe all autumn results as invalid or ban the test outright. It emphasizes its usefulness for identifying negative cases in July–December and considering repeat testing when signs and results disagree. A high autumn response should not be classified using the January–June positive threshold.

Preparation and scheduling belong to the clinic's protocol. Tell the team about recent feeding, sedation, transport, and any planned insulin testing; ask for its instructions before the appointment. The EEG allows testing after hay, distinguishes recent grain feeding, and specifies timing relative to an oral sugar test. The EEG also describes transient coughing, flehmen responses, and yawning after TRH. The treating team should explain what to expect for the individual horse.

Why laboratories can give different ACTH answers

Different analyzers do not necessarily report interchangeable concentrations. The EEG notes that the Immulite 2000xpi assay cross-reacts to some extent with CLIP, a related pituitary peptide that can also increase in PPID. The Tosoh AIA-900 assay does not cross-react with CLIP and uses lower cutoffs. That difference cannot be corrected by choosing whichever published range gives the preferred answer.

The 2026 study by Durham and Lopes compared contemporary platforms using surplus clinical plasma samples and CLSI transference methods. Its abstract reports close agreement between Immulite 1000 and 2000XPi, but a nonlinear, concentration-dependent difference between Immulite 2000XPi and Tosoh AIA-900. Tosoh AIA-360 and AIA-900 agreed closely. Platform-specific thresholds were generated for four seasonal periods. This supports asking for an assay-specific interpretation; it does not supply a universal conversion factor. Detailed transferred threshold values are not reproduced here.

Published guidance also differs by method, purpose, and date. The New Mexico Veterinary Diagnostic Services article lists typical normal values below 35 pg/mL for chemiluminescent testing or below 45–50 pg/mL for radioimmunoassay and notes seasonal effects. Its references span 2003–2011. The Merck Veterinary Manual page gives post-TRH categories of under 110, 110–200, and over 200 pg/mL, alongside a January–July evocative-testing window. The EEG uses January–June categories and more qualified July–December interpretation. These sources are not interchangeable tables for a single result; their differences are a reason to ask which guidance the laboratory uses.

Before questioning the result, review the sample history

Handling problems can distort ACTH results, but the direction of the error is not always the same. Cornell notes instability associated with heat and time on blood cells. The EEG specifically warns that freezing an uncentrifuged sample can produce spuriously high values. A suspect result therefore cannot automatically be explained as either 'too low from shipping' or 'too high from stress.'

Ask the clinic whether the sample met the receiving laboratory's requirements. This is a review of the clinic's collection and submission record, rather than a home blood-collection checklist. Two primary protocols illustrate why the destination matters:

The different separation windows are published protocol differences, not permission to ignore the laboratory accepting the sample. If the collection record is incomplete, the veterinarian can ask that laboratory whether the result remains interpretable or whether another sample would help. The owner need not infer sample quality from the numerical result alone.

Biological context also belongs in that review. The EEG cautions about recent trailering, visible excitement, sedation, and severe uncontrolled pain. Its guidance allows testing in laminitic horses and does not treat all pain as a reason to postpone testing. Tell the veterinarian what happened around the appointment; the team can decide whether timing or concurrent illness changes interpretation.

Breed can shift the background expectation. EEG guidance cautions that healthy Arabians and donkeys may have higher ACTH concentrations in any season, while Welsh and Shetland ponies may have higher autumn values. This does not make those animals immune to PPID, nor does it justify inventing a breed correction. It increases the importance of clinical signs and an appropriate laboratory interpretation.

Inside the interpretive zone: what happens next?

For the hypothetical gelding with a September baseline of 47 pg/mL, the useful conclusion is 'indeterminate under this published assay-specific scheme.' It is not 'borderline-positive,' and it is not reassurance that every change is normal aging. The veterinarian needs to reconcile the report with delayed shedding, muscle change, performance, infections, and any other findings.

The EEG diagnostic algorithm recommends testing horses with compatible clinical signs and does not recommend testing in their absence. In older horses with advanced characteristic signs, a clinical diagnosis may sometimes be made without testing. Conversely, a value in the unlikely range does not erase convincing signs: false-positive and false-negative results are possible.

The guidance includes reassessment in 3–6 months when findings are uncertain. Depending on the horse, the veterinarian may choose monitoring, repeat baseline testing, a TRH test with the appropriate seasonal interpretation, or a treatment trial. January–June can provide a useful TRH testing window, but it is not a universal requirement to leave every horse untreated until winter. A future result above 200 pg/mL is also not guaranteed in early PPID.

Insulin status is a related question with its own testing requirements. The EEG recommends assessing insulin dysregulation alongside PPID evaluation. An ACTH concentration alone does not establish insulin control or explain every laminitis episode. If the horse develops new lameness or suspected laminitis, contact the veterinarian promptly rather than waiting for a planned endocrine recheck.

If pergolide starts, keep the calendar in the monitoring record

Pergolide is the main medical treatment discussed in the EEG guidance. The veterinarian's treatment plan should state when to reassess signs and whether endocrine testing is needed. The EEG recommends evaluation 1–3 months after treatment begins and then every 6–12 months, with the schedule adapted to the response. Reduced appetite can occur after starting treatment or increasing a dose; report it to the prescribing veterinarian for advice.

For longer-term comparisons, record the month, assay, units, test type, medication history, and clinical observations with each result. The EEG emphasizes comparison in the same season from year to year. A fall from 120 pg/mL in October to 28 pg/mL in March could reflect treatment, the seasonal decline, or both. Those two numbers alone cannot isolate the medication's effect.

Clinical improvement is the EEG's most important indicator of treatment response. ACTH may remain above the unlikely range even when a horse improves; that does not automatically require more medication. Cornell also notes that post-treatment TRH reference ranges have not been established. Diagnostic cutoffs should therefore not become an owner-directed treatment target. The clinician decides how signs and laboratory findings should guide monitoring or changes.

Five questions to take to the results conversation

Keep the original laboratory report, rather than only a photograph of the flagged number. Ask the clinic to clarify missing method or timing information and write down the follow-up plan. These questions connect the laboratory details to the decision about this horse:

1. Which analyzer, units, and seasonal interpretation did this laboratory use, and is its printed boundary a reference limit or a diagnostic threshold?

2. Does this value belong to a resting baseline sample, or a correctly identified 10- or 30-minute post-TRH sample?

3. Did sample handling meet this laboratory's protocol, and could transport, sedation, stress, illness, or breed affect interpretation?

4. Which examination findings support PPID, and does my horse also need assessment for insulin dysregulation?

5. What is the next step and its timing: monitoring, repeat testing, further testing, or veterinarian-directed treatment, and which changes should prompt an earlier call?

Sources

The decision zones are attributed to the EEG's October 2025 consensus recommendations. The EEG discloses Boehringer Ingelheim support for participant travel and states that the company does not influence its recommendations; it also discloses a member's affiliation with a laboratory offering endocrine testing. The JAVMA cohort is healthy-horse seasonality evidence, the 2026 platform study is used at abstract level, and the 2022 article is a narrative review. Cornell, New Mexico VDS, and Merck illustrate named protocols and interpretation differences.

• Equine Endocrinology Group: Recommendations for the Diagnosis and Management of PPID — revised October 2025; Tables 2, 3, 5, and 6 and the diagnosis and monitoring algorithms.

• Cornell AHDC: Equine PPID / Cushing’s Testing — specimen and TRH protocols; page revision code March 2018.

• Pinn-Woodcock et al.: Seasonal elevation in equine ACTH throughout the contiguous United States — JAVMA, 2025; 263:1380–1389.

• Durham and Lopes: Transference of diagnostic thresholds for equine plasma ACTH — Domestic Animal Endocrinology, 2026; 96:107015; abstract.

• Kirkwood, Hughes, and Stewart: Pituitary Pars Intermedia Dysfunction (PPID) in Horses — Veterinary Sciences, 2022; 9:556; narrative review.

• New Mexico VDS: Diagnostic Criteria for Equine PPID and Differentiation with EMS — laboratory guidance with references from 2003–2011; used as historical method context.

• Merck Veterinary Manual: Hypertrichosis Associated With Adenomas of the Pars Intermedia in Horses — reference-manual comparison of post-TRH interpretation and testing season.