Thyrotropin-releasing hormone (TRH) testing is a relatively easy and expedient method of identifying horses with PPID. The TRH test has superior sensitivity and specificity compared to simply looking at baseline adrenocorticotropin (ACTH) concentrations, especially in times of seasonal fluctuations.
The test calls for taking a baseline blood sample, followed by another 10 or 30 minutes later to compare ACTH concentrations following intravenous administration of 1 mg of TRH. Researchers from Australia and the Netherlands evaluated different time points of ACTH measurement following TRH administration.
Study on Sampling Times for TRH Testing
In a 2022 study, Thane et al. found that even a one-minute alteration in sampling time for the 10-minute protocol had a significant effect (differing by ≥ 10%) on ACTH concentrations in three-quarters of tested horses. ACTH concentrations also increase rapidly in the first 15 minutes following TRH administration and are more stable around 30 minutes. The objective of the current study was to see if less-than-precise sampling times could also affect the 30-minute protocol.
Study Results
The study involved 15 control horses with no clinical signs of PPID and 12 PPID horses. Blood was drawn for ACTH measurement at 9, 10, 11, 29, 30, and 31 minutes following IV administration of 1 mg TRH. The results are important:
- The researchers found no significant effect on imprecise sampling time for absolute ACTH concentrations for either the 10- or 30-minute protocols when samples were collected early or late by only one minute. However, imprecision of more than a minute had more impactful effects on the 10-minute protocol.
- Based on cutoff values established by the Equine Endocrinology Group, ACTH > 220 pg/ml is considered positive, 110-220 pg/ml is equivocal, and < 110 pg/ml is negative. All 12 horses were considered PPID-positive with the 30-minute protocol, whereas 6/12 were classified as equivocal or negative using the 10-minute protocol.
Disagreement in diagnosis occurred between the 10- and 30-minute protocols. Half of the PPID-positive horses at the 30-minute protocol would not have been classified as positive in the 10-minute protocol unless clinical signs were taken into account. Supportive clinical signs can lead a practitioner to consider horses with equivocal results to be positive for PPID.
Final Thoughts
The authors stated, “Precise sampling time, use of correct diagnostic cutoff values, and contextual interpretation of ACTH concentrations remain fundamental for the diagnosis of PPID.”
Reference
Vorster DM, Wang W, Kemp KL, et al. Clinical implications of imprecise sampling time for 10- and 30-min thyrotropin-releasing hormone stimulation tests in horses. Equine Veterinary Journal August 2023; DOI: 10.1111/evj.13991
